Add an execution mode to OpenMCPAuthProxy to act as a Identity Gateway to validate tokens issued by a trusted Auth Server

[ayshsandu]

It is suggested that, "MCP specification DOES NOT require MCP server developers to implement their own authorization server (AS) and instead relies on developers adopting dedicated and well-tested AS implementations." - https://github.com/modelcontextprotocol/modelcontextprotocol/pull/284/files

In such cases, MCP Servers still need to validate the tokens presented by the client. By doing $subject, MCP Server developers can use OpenMCPAuthProxy to implement simple authorization capabilities to MCP Server.