feat(pkce): add usePkce custom extension

[maximilianfalco]

🚥 Resolves ISSUE_ID

🧰 Changes

This PR adds support for PKCE (Proof Key for Code Exchange) in OAuth authorization code flows by introducing a new usePkce property within the oauth-options extension.

• Added usePkce?: boolean property to the OAUTH_OPTIONS extension interface
• When enabled, OAuth authorization code flows should use PKCE with auto-generated code verifier and challenge
• When disabled (default), uses standard OAuth 2.0 authorization code flow with client credentials

Usage:

{
  "x-readme": {
    "oauth-options": {
      "usePkce": true
    }
  }
}

🧬 QA & Testing

• Added unit tests for root-level and operation-level usePkce configuration
• Added tests for multiple OAuth options working together
• All existing extension tests continue to pass

[erunion]

very minor request but otherwise lgtm

[erunion]

@maximilianfalco I'll work on getting this published and set for deployment today but can you handle updating our docs? https://docs.readme.com/docs/openapi-extensions