refactor: introduce cached schema extension metadata

[jterapin]

Context

Shape and member metadata is currently resolved through multiple lookups and separate cache entries. This causes parsers, builders, and protocol code to rediscover related metadata.

What changed

• Added cached, array-backed extension metadata for Schema, JSON, XML, CBOR, and HTTP.
• Preserved raw traits on shapes and members for dynamic-client support.
• Added explicit for(shape) and for_member(member) resolution paths.
• Centralized outer extension slots while keeping inner slots local to each extension.
• Bundled target classes, member metadata, traits, wire names, and indexes.
• Standardized list/map member construction through add_member.
• Regenerated the smithy-ruby weather and shapes projections.
• Added the initial HTTP extension metadata foundation.

Out of scope

• Migrating all remaining protocol and codec consumers.
• Codec-specific hot-path optimizations.
• Broad staging service regeneration.

Testing

• Focused Schema, JSON, XML, CBOR, and HTTP specs pass:
  613 examples, 0 failures.
• Regenerated projections load successfully.
• RuboCop passes for the affected files.