docs: refresh README after library analysis

[Copilot]

Summary

Reviewed the library API and refreshed README.md where documentation was outdated.

Changes

• Updated Quick Start dependency versions:
  • rig-openapi-tools to 0.1.6
  • rig-core to 0.34
• Expanded the Builder method table to include existing public API options:
  • .api_key_header(name, key)
  • .api_key_query(name, key)
  • .basic_auth(username, password)

Validation

• cargo check
• cargo test
• cargo clippy -- -D warnings
• cargo fmt --check
• Parallel validation: Code Review ✅, CodeQL ✅ (trivial docs-only changes)