Add const keyword and serialization logic

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -50,6 +50,11 @@ public static class OpenApiConstants
         /// </summary>
         public const string Title = "title";
 
+        /// <summary>
+        /// Field: Const
+        /// </summary>
+        public const string Const = "const";
+
         /// <summary>
         /// Field: Type
         /// </summary>

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -83,6 +83,11 @@ public class OpenApiSchema : IOpenApiAnnotatable, IOpenApiExtensible, IOpenApiRe
         /// </summary>
         public virtual JsonSchemaType? Type { get; set; }
 
+        /// <summary>
+        /// Follow JSON Schema definition: https://json-schema.org/draft/2020-12/json-schema-validation
+        /// </summary>
+        public virtual string Const { get; set; }
+
         /// <summary>
         /// Follow JSON Schema definition: https://tools.ietf.org/html/draft-fge-json-schema-validation-00
         /// While relying on JSON Schema's defined formats,
@@ -347,6 +352,7 @@ public OpenApiSchema(OpenApiSchema schema)
         {
             Title = schema?.Title ?? Title;
             Id = schema?.Id ?? Id;
+            Const = schema?.Const ?? Const;
             Schema = schema?.Schema ?? Schema;
             Comment = schema?.Comment ?? Comment;
             Vocabulary = schema?.Vocabulary != null ? new Dictionary<string, bool>(schema.Vocabulary) : null;
@@ -563,6 +569,7 @@ internal void WriteV31Properties(IOpenApiWriter writer)
             writer.WriteProperty(OpenApiConstants.Id, Id);
             writer.WriteProperty(OpenApiConstants.DollarSchema, Schema);
             writer.WriteProperty(OpenApiConstants.Comment, Comment);
+            writer.WriteProperty(OpenApiConstants.Const, Const);
             writer.WriteOptionalMap(OpenApiConstants.Vocabulary, Vocabulary, (w, s) => w.WriteValue(s));
             writer.WriteOptionalMap(OpenApiConstants.Defs, Definitions, (w, s) => s.SerializeAsV31(w));
             writer.WriteProperty(OpenApiConstants.DynamicRef, DynamicRef);

src/Microsoft.OpenApi/Models/References/OpenApiSchemaReference.cs

@@ -90,6 +90,8 @@ internal OpenApiSchemaReference(OpenApiSchema target, string referenceId)
         /// <inheritdoc/>
         public override JsonSchemaType? Type { get => Target.Type; set => Target.Type = value; }
         /// <inheritdoc/>
+        public override string Const { get => Target.Const; set => Target.Const = value; }
+        /// <inheritdoc/>
         public override string Format { get => Target.Format; set => Target.Format = value; }
         /// <inheritdoc/>
         public override string Description