feat: check for PKCE of authorization server metadata

[tnorimat]

Motivation and Context

Described in #207 .

How Has This Been Tested?

• npm test (Test Files 6 passed, Tests 89 passed)
• npm test -- src/scenarios/authorization-server/authorization-server-metadata.test.ts (Test Files 1 passed, Tests 3 passed)
• npm run start -- authorization --url http://localhost:8080/.well-known/oauth-authorization-server/realms/mp
  --url URL of the authorization server metadata endpoint (using keycloak 26.5.7)
• npm run start -- authorization --url http://localhost:8080/.well-known/oauth-authorization-server/realms/mp
  --url URL of the authorization server metadata endpoint (using keycloak 4.0.0.Beta2, which does not support PKCE metadata, before merging KEYCLOAK-7451 OAuth Authorization Server Metadata for Proof Key for Code Exchange keycloak/keycloak#5228 )

Breaking Changes

Nothing

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/@modelcontextprotocol/conformance@212

commit: 1782dcb