Adding QOI metadata and constants

I'm starting to need help because I don't understand the project structure

Everything I added was taken from the specification and structured similarly to PNG and BMP

Author: LuisAlfredo92

src/ImageSharp/Formats/Qoi/QoiChannels.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+/// <summary>
+/// Provides enumeration of available QOI color channels.
+/// </summary>
+public enum QoiChannels
+{
+    /// <summary>
+    /// Each pixel is an R,G,B triple.
+    /// </summary>
+    Rgb = 3,
+
+    /// <summary>
+    /// Each pixel is an R,G,B triple, followed by an alpha sample.
+    /// </summary>
+    Rgba = 4
+}

src/ImageSharp/Formats/Qoi/QoiColorSpace.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+// ReSharper disable InconsistentNaming
+// ReSharper disable IdentifierTypo
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+/// <summary>
+/// Enum for the different QOI color spaces.
+/// </summary>
+public enum QoiColorSpace
+{
+    /// <summary>
+    /// sRGB color space with linear alpha value
+    /// </summary>
+    SRGB_WITH_LINEAR_ALPHA,
+
+    /// <summary>
+    /// All the values in the color space are linear
+    /// </summary>
+    ALL_CHANNELS_LINEAR
+}

src/ImageSharp/Formats/Qoi/QoiConstants.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Text;
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+internal static class QoiConstants
+{
+
+    /// <summary>
+    /// Gets the bytes that indicates the image is QOI
+    /// </summary>
+    public static ReadOnlySpan<byte> Magic => Encoding.UTF8.GetBytes("qoif");
+
+    /// <summary>
+    /// The list of mimetypes that equate to a QOI.
+    /// See <seealso cref="https://github.com/phoboslab/qoi/issues/167"/>
+    /// </summary>
+    public static readonly IEnumerable<string> MimeTypes = new[] { "image/qoi", "image/x-qoi", "image/vnd.qoi" };
+
+    /// <summary>
+    /// The list of file extensions that equate to a QOI.
+    /// </summary>
+    public static readonly IEnumerable<string> FileExtensions = new[] { "qoi" };
+}

src/ImageSharp/Formats/Qoi/QoiHeader.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Text;
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+/// <summary>
+/// Represents the qoi header chunk.
+/// </summary>
+internal readonly struct QoiHeader
+{
+    public QoiHeader(uint width, uint height, QoiChannels channels, QoiColorSpace colorSpace)
+    {
+        this.Width = width;
+        this.Height = height;
+        this.Channels = channels;
+        this.ColorSpace = colorSpace;
+    }
+
+    /// <summary>
+    /// Magic bytes "qoif"
+    /// </summary>
+    public byte[] Magic { get; } = Encoding.UTF8.GetBytes("qoif");
+
+    /// <summary>
+    /// Image width in pixels (BE)
+    /// </summary>
+    public uint Width { get; }
+
+    /// <summary>
+    /// Image height in pixels (BE)
+    /// </summary>
+    public uint Height { get; }
+
+    /// <summary>
+    /// Color channels of the image. 3 = RGB, 4 = RGBA.
+    /// </summary>
+    public QoiChannels Channels { get; }
+
+    /// <summary>
+    /// Color space of the image. 0 = sRGB with linear alpha, 1 = All channels linear
+    /// </summary>
+    public QoiColorSpace ColorSpace { get; }
+}

src/ImageSharp/Formats/Qoi/QoiMetadata.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Text;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+/// <summary>
+/// Provides Qoi specific metadata information for the image.
+/// </summary>
+public class QoiMetadata : IDeepCloneable
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="QoiMetadata"/> class.
+    /// </summary>
+    public QoiMetadata()
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="QoiMetadata"/> class.
+    /// </summary>
+    /// <param name="other">The metadata to create an instance from.</param>
+    public QoiMetadata(QoiMetadata other)
+    {
+        this.Width = other.Width;
+        this.Height = other.Height;
+        this.Channels = other.Channels;
+        this.ColorSpace = other.ColorSpace;
+    }
+
+    /// <summary>
+    /// Gets or sets image width in pixels (BE)
+    /// </summary>
+    public uint Width { get; set; }
+
+    /// <summary>
+    /// Gets or sets image height in pixels (BE)
+    /// </summary>
+    public uint Height { get; set; }
+
+    /// <summary>
+    /// Gets or sets color channels of the image. 3 = RGB, 4 = RGBA.
+    /// </summary>
+    public QoiChannels Channels { get; set; }
+
+    /// <summary>
+    /// Gets or sets color space of the image. 0 = sRGB with linear alpha, 1 = All channels linear
+    /// </summary>
+    public QoiColorSpace ColorSpace { get; set; }
+
+    /// <inheritdoc/>
+    public IDeepCloneable DeepClone() => new QoiMetadata(this);
+}