Adds a license SPDX identifier

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/V3/OpenApiLicenseDeserializer.cs

@@ -22,6 +22,12 @@ internal static partial class OpenApiV3Deserializer
                     o.Name = n.GetScalarValue();
                 }
             },
+            {
+                "identifier", (o, n) =>
+                {
+                    o.Identifier = n.GetScalarValue();
+                }
+            },
             {
                 "url", (o, n) =>
                 {

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -120,6 +120,11 @@ public static class OpenApiConstants
         /// </summary>
         public const string Name = "name";
 
+        /// <summary>
+        /// Field: Identifier
+        /// </summary>
+        public const string Identifier = "identifier";
+
         /// <summary>
         /// Field: Namespace
         /// </summary>

src/Microsoft.OpenApi/Models/OpenApiLicense.cs

@@ -19,6 +19,11 @@ public class OpenApiLicense : IOpenApiSerializable, IOpenApiExtensible
         /// </summary>
         public string Name { get; set; }
 
+        /// <summary>
+        /// An SPDX license expression for the API. The identifier field is mutually exclusive of the url field.
+        /// </summary>
+        public string Identifier { get; set; }
+
         /// <summary>
         /// The URL pointing to the contact information. MUST be in the format of a URL.
         /// </summary>
@@ -40,6 +45,7 @@ public OpenApiLicense() {}
         public OpenApiLicense(OpenApiLicense license)
         {
             Name = license?.Name ?? Name;
+            Identifier = license?.Identifier ?? Identifier;
             Url = license?.Url != null ? new Uri(license.Url.OriginalString) : null;
             Extensions = license?.Extensions != null ? new Dictionary<string, IOpenApiExtension>(license.Extensions) : null;
         }
@@ -72,6 +78,9 @@ private void WriteInternal(IOpenApiWriter writer, OpenApiSpecVersion specVersion
             // name
             writer.WriteProperty(OpenApiConstants.Name, Name);
 
+            // identifier
+            writer.WriteProperty(OpenApiConstants.Identifier, Identifier);
+
             // url
             writer.WriteProperty(OpenApiConstants.Url, Url?.OriginalString);