Add FromFormOrJsonModelBinder for mixed multipart/form-data and JSON support

[brunohash]

Fixes #61856

This PR introduces a generic model binder (FromFormOrJsonModelBinder<T>) that enables binding from both application/json and multipart/form-data request types into a single model.

Key additions:

• FromFormOrJsonModelBinder<T> – the core binder
• FromFormOrJsonAttribute – attribute to annotate parameters
• FromFormOrJsonModelBinderProvider – resolves the correct generic type
• IFormFileReceiver – optional interface for models to receive files
• Unit test for JSON payload support

This is useful for modern APIs that need to support clients submitting structured data along with file uploads.

Let me know if you'd like to see additional test coverage or refinements.

[brunohash]

@dotnet-policy-service agree

[brunohash]

Hi @sebastienros — this PR introduces a lightweight generic binder to support both JSON and multipart requests. Let me know if it's something the team would consider. Happy to adapt based on feedback. Thanks!

[gfoidl]

Thanks for your interest and the contribution!

But as this adds members to the public APIs we have to follow the API review process.
You already did the first step, by opening the issue (can you edit the issue to follow the API Proposal issue template please?).

When API review is done, we can continue with your PR.
In the meantime I suggest that either you put the PR to draft or close it (and re-open it later in case of succesful API review).

Anyway, thanks for your effort.