feat: add extra_claims support to TokenAuthority

Implements extra claims support in the Python TokenAuthority class to match the Ruby JsonWebToken implementation.

Changes

• Add optional extra_claims parameter to TokenAuthority.encode() method
• Merge extra claims into JWT payload using claims.update(extra_claims)
• Add comprehensive test case test_token_authority_encode_with_extra_claims() to verify functionality

Motivation

The Ruby implementation already supports adding custom claims to JWT tokens via the extra_claims parameter. This change brings feature parity to the Python implementation, allowing callers to include additional custom claims in the token payload.