Add MPFD Support

[jorgerangel-msft]

This PR adds support for generating convenience client methods for multipart/form-data request.

contributes to : #9548

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commit: b4407ea

[JoshLove-msft]

Reviewed the production source (emitter TS, Input layer, ClientModel/Core generators). Overall the MPFD design is sound and well-tested. Two inline notes below: one medium correctness concern about mutating a shared cached type in the emitter, and one minor dead-branch nit. Everything else (multipart serialization gating, optional/required handling, isMulti per-element emission, Input converters + Json-usage rule, SCME0004 Experimental gating, constructor augmentation, optional-body MediaType forwarding) looks correct.

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[JoshLove-msft]

Re-reviewed after the pr feedback commit (0dcdf53). Both of my earlier comments are resolved — thanks:

• The dead stream.Position > int.MaxValue branch in MultipartFormDataHelperDefinition.cs is gone; the helpers now return BinaryData.FromBytes(stream.GetWrittenMemory(), mediaType) directly. ✅
• The shared-bytes cache mutation is fixed by cloning, and the new does not mutate cached bytes types when marking a multipart bytes file part test locks in the invariant (asserting no cached bytes carries isFileType). ✅ Nice regression test.

One residual, inline: the same mutation-of-shared-cache pattern still applies to the model and array-of-model branches, which were not cloned.

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