Move header helpers to metadata

[samuel-williams-shopify]

Move the remaining header and metadata helpers out of the deprecated Protocol::GRPC::Methods namespace.

• Add Protocol::GRPC::Metadata.build for constructing gRPC request headers with application metadata, timeout, and content type.
• Add Protocol::GRPC::Metadata.extract for extracting application metadata and decoding binary values.
• Keep Methods.build_headers and Methods.extract_metadata as deprecated compatibility wrappers.
• Update Call, internal tests, guides, and design documentation to use Metadata directly.
• Keep Protocol::GRPC::Header focused on typed header values and HEADER_POLICY.

Validation:

• bundle exec bake test
• bundle exec rubocop (58 files, no offenses)
• bundle exec bake decode:index:coverage lib (106/106 public definitions documented)