refactor: establish cached Smithy HTTP binding metadata

[jterapin]

Summary

Establishes cached Smithy HTTP binding metadata for later staging REST runtime adoption.

• Adds Smithy::Client::HttpExtension for operation method, path, static query, and response-code metadata.
• Adds cached header, prefix-header, query, query-params, label, document-body, payload, and response-code bindings.
• Resolves payload content types through Schema metadata.
• Documents the HTTP extension API and adds focused behavior coverage.

Out of scope

• Staging REST request and response consumer migration.
• A second HttpBinding compatibility facade.
• Legacy compatibility removal and generated output.

Validation

• Focused HTTP extension spec: 2 examples, 0 failures.
• Full smithy-client RBS suite: 839 examples, 0 failures, 2 pending.
• RuboCop and git diff --check passed.