Add parsers for common content types.

[samuel-williams-shopify]

Summary

• replace the representation object with explicit Parser#parse(content_type, input) dispatch
• add bounded JSON, URL-encoded form, and multipart form parsers
• provide an explicitly required, frozen Protocol::Content::Parser.default registry
• forward caller blocks for incremental parsing and streaming multipart uploads
• translate format-specific parser limits into ContentTooLargeError while retaining the original cause
• document parser usage in a dedicated Getting Started guide

Callers retain ownership of message metadata, body adaptation, consumption, and memoization. Parsers receive a readable IO-like input directly; multipart boundary extraction is handled by content-type dispatch. Multipart uploads remain explicitly streaming and do not impose tempfile semantics.

Verification

• 22 tests, 31 assertions
• 100% line coverage
• RuboCop clean
• Decode: 17/17 public definitions documented
• tested with protocol-url 0.9.0 and protocol-multipart 0.5.0