Supporting Azure Identity for Authentication

[SolomonTomer]

For MCP servers that rely on Azure identity for authentication (MSAL, MISE, etc), this PR adds support for interactive login when the local MCP proxy is starting, sending the generated JWT token for the given audience, in every request.
This enables remote HTTP MCP servers to fully authorize Azure tokens within each request.

[geelen]

Hmm, this is a pretty heavy change to support a single auth strategy. But I've enabled pkg.pr.new builds so if you fix the conflicts you should get a URL you can do some testing with. I'd love to see a little screencast of what this enables..

My only thought to make this less invasive would be something like adding a --generate-env or --token-cmd flag to mcp-remote, and then a new standalone CLI like npx generate-azure-tokens or something. So you make mcp-remote know basically nothing about azure, it just has some way of shelling out to a custom command that does? Idk if that would work though...