Add enumerations support

Author: Havret

src/System.Text.Json.Protobuf/ProtobufConverter.cs

@@ -143,6 +143,7 @@ private Type GetFieldType(FieldDescriptor fieldDescriptor, Dictionary<string, Ty
                 return typeof(string);
             case FieldType.Message when fieldDescriptor.MessageType.ClrType != null:
                 return fieldDescriptor.MessageType.ClrType;
+            case FieldType.Enum:
             case FieldType.Message:
                 return propertyTypeLookup[fieldDescriptor.PropertyName];
             default:

test/System.Text.Json.Protobuf.Tests/MessageWithEnumFieldTests.Should_serialize_message_with_enum_field.approved.json

@@ -0,0 +1,3 @@
+{
+  "enumField": 1
+}

test/System.Text.Json.Protobuf.Tests/MessageWithEnumFieldTests.cs

@@ -0,0 +1,46 @@
+using System.Text.Json.Protobuf.Tests.Utils;
+using Shouldly;
+using SmartAnalyzers.ApprovalTestsExtensions;
+using Xunit;
+
+namespace System.Text.Json.Protobuf.Tests;
+
+public class MessageWithEnumFieldTests
+{
+    [Fact]
+    public void Should_serialize_message_with_enum_field()
+    {
+        // Arrange
+        var msg = new MessageWithEnum
+        {
+            EnumField = TestEnum.FirstOption
+        };
+        var jsonSerializerOptions = TestHelper.CreateJsonSerializerOptions();
+ 
+        // Act
+        var serialized = JsonSerializer.Serialize(msg, jsonSerializerOptions);
+
+        // Assert
+        var approver = new ExplicitApprover();
+        approver.VerifyJson(serialized);
+    }
+
+    [Fact]
+    public void Should_deserialize_message_with_with_enum_field()
+    {
+        // Arrange
+        var msg = new MessageWithEnum
+        {
+            EnumField = TestEnum.FirstOption
+        };
+        var jsonSerializerOptions = TestHelper.CreateJsonSerializerOptions();
+
+        // Act
+        var serialized = JsonSerializer.Serialize(msg, jsonSerializerOptions);
+        var deserialized = JsonSerializer.Deserialize<MessageWithEnum>(serialized, jsonSerializerOptions);
+
+        // Assert
+        deserialized.ShouldNotBeNull();
+        deserialized.ShouldBeEquivalentTo(msg);
+    }
+}

test/System.Text.Json.Protobuf.Tests/Protos/message_with_enum.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+option csharp_namespace = "System.Text.Json.Protobuf.Tests";
+
+message MessageWithEnum {
+  TestEnum enum_field = 1;
+}
+
+enum TestEnum {
+  UNKNOWN = 0;
+  FIRST_OPTION = 1;
+  SECOND_OPTION = 2;
+}