v2.1.0: - Add prepare/apply API for remote HTTP message signing

• Add prepare/apply API for remote HTTP message signing

• Honor and validate HMAC shared secret arguments

• Reject scalar strings as covered component sequences

• Reject prohibited control characters in covered header values

• Preserve empty OPTIONS paths in the target URI

• Use the RFC query-param component name

• Allow strict serialization with dictionary member selection

• Reject non-uppercase request methods before signature construction

• Report verification input and key failures as InvalidSignature

• Enforce Structured Field types and derived component characters

• Reject duplicate signature labels through the Structured Fields parser

• Normalize the full obsolete header folding sequence

• Compare signature timestamps in UTC across DST transitions

• Require request context parameters on response signatures

• Normalize signature component values per RFC 9421

• Test and documentation improvements