Bug description

In v2, [MessagePackObject] wasn't required for code generation to be able to serialize and deserialize the base type.
In v3, a field or property which includes [Union] but does not include [MessagePackObject] will not be serialized or deserialized in an AOT-compiled build; a MessagePack.FormatterNotRegisteredException is thrown indicating that the base type is not recognized.

Repro steps

1. Create a new Unity project with MessagePack v3
2. Configure the project to use IL2CPP
3. Create an abstract base class with a [Union] attribute
4. Instantiate a derived instance of that type at runtime.
5. Serialize and deserialize it using an abstract reference.
6. --> FormatterNotRegisteredException is thrown

Expected behavior

Either an analyzer should indicate that [MessagePackObject] is required on the abstract base class, or the attribute should not be necessary (similar to it not being needed for interfaces).

Actual behavior

A MessagePack.FormatterNotRegisteredException was thrown when attempting to serialize or deserialize an field or property of the abstract type.

• Version used: 3.1.4
• Runtime: Unity IL2CPP