Support OneOf Validation with FieldMasks

[KrishnaIyer]

Summary:

Protobuf OneOf messages are to be validated based on the fieldmask provided.

Why do we need this?

Since we use oneof messages in the codebase.

What is already there? What do you see now?

No support for OneOfs with fieldmasks.

What is missing? What do you want to see?

Support OneOfs with fieldmasks.

How do you propose to implement this?

Modify the existing util function to inject fields specific to the actually defined message.

What can you do yourself and what do you need help with?

I can create a PR.