Support adding env when adding new mcp servers

[gyliu513]

There is no place to set up some env vars, for some integration like GitHub, it may request some env for api key etc.

[NathanTarbert]

@gyliu513, if you put in the SSE URL, you will be prompted to authenticate. This happens with Reddit, Slack, etc.

If you want to add this functionality to your app, just configure it so you can add an API key or token