Support Authorization Server Metadata with path insertion and endpoints on subpaths

[wille]

Motivation and Context

Fixes #167

This PR adds support for Authorization Server Metadata with path insertion and endpoints on subpaths

One might not want to run their authorization server on the root of their domain (like https://api.example.com) but instead run it on a path (https://api.example.com/oauth)

How Has This Been Tested?

• A unit test was added
• The example simpleOAuthClient.ts now works properly with the DEFAULT_SERVER_URL (http://localhost:3000/mcp) if testing on a mcpAuthRouter

Breaking Changes

If the user has configured their mcpAuthRouter with a issuerUrl or baseUrl that contains a path (like https://localhost/path), endpoints will now be setup at https://localhost/path/authorize instead of https://localhost/authorize like it works right now, even if the user has set a path.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

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

I used https://modelcontextprotocol.io/specification/draft/basic/authorization#authorization-server-metadata-discovery as a reference along with my observations of my own implementation of the MCP OAuth spec

[pkg-pr-new]

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npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1095

commit: 117fc07