ACP: support mcp config passed session/new

[devm33]

https://agentclientprotocol.com/protocol/session-setup#mcp-servers

[cedricvidal]

Here's an example of a ACP client demonstrating the requested feature:
https://github.com/cedricvidal/copilot-acp-mcp-issue/blob/main/client.ts

[josevalim]

This might be fixed as well. :)