Add support for multiple resources per MCP Authentication scheme

Currently, the McpAuthenticationHandler only supports responding with a single "resource" URI per scheme name as defined in https://www.rfc-editor.org/rfc/rfc9728.html. This means that you can only use OAuth for one of either the Streamable HTTP endpoint ("/") or the SSE endpoint ("/sse"), but not both. Technically, the new auth spec only applies to Streamable HTTP since that's the only HTTP transport defined in the 2025-06-18 spec and the 2025-03-26 spec for that matter, but the currently neither the server or the client code on the csharp-sdk treat SSE any differently than Streamable HTTP.

Furthermore, it be nice to be able to define multiple Streamable HTTP endpoints on different paths on a single HTTP server. However, this is something we should look at improving in general, since configuring the IMcpServerBuilder and/or McpServerOptions in the typical way doesn't allow multitenancy, and using ConfigureSessionOptions is a bit awkward.

https://modelcontextprotocol.io/specification/2025-06-18/basic/authorization

Start by reading McpAuthenticationHandler and the authorization specification linked in the issue to understand how one resource is currently associated with each scheme. Then inspect IMcpServerBuilder, McpServerOptions, and ConfigureSessionOptions to compare configuration paths; done means supporting multiple resources and endpoints without breaking existing Streamable HTTP or SSE behavior.