[DO NOT MERGE]

[Copilot]

Adds GetReadWriteMetadataSize and FillReadWriteMetadata from #131027 without removing existing metadata infrastructure or DacDbi APIs.

• Native DAC
  • Serializes writable metadata into a contiguous ECMA-335 image.
  • Implements size-query and caller-provided-buffer APIs.
• cDAC
  • Exposes writable metadata serialization through IEcmaMetadata.
  • Implements matching managed DacDbi entry points.

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