Support MCP registry endpoint as org-policy to allow-list MCP

Allow enterprises to set a MCP registry endpoint and registry restriction in GH org policies. VS Code should read these policies and provide browsing and installing MCP servers from this registry. If restriction is enabled then prevent starting MCP servers that are not in the registry.

• The registered MCP server should support the v0.1 spec of MCP registry spec
• The configured end point should be of the form https://api.mcp.github.com/. VS Code will add necessary routes like v0.1/servers/ to access data from the server.
• If access is restricted to registry (in the policy) then
  • Allow starting the installed server only if it is in the registry
  • If the server is a remote server then allow only if the remote server url is same as in the registry