Implement selective tools activation in gptel-mcp-connect

[vspinu]

This MR allows registration of MCP servers with selective tool activation. Currently one can only activate all tools for a server. My personal use case is to make all MCPs available to gptel, but don't activate any of them globally.

I am also adding a short line to the README as it take a bit of digging currently to figure this server initialization part.

[karthink]

Thanks for the PR @vspinu! Busy with some features, will take a look soon.

[karthink]

@vspinu Reviewing this PR now.

1. I take it there is no change to the interactive behavior of gptel-mcp-connect?
2. Why the specification ("server-name" . t)? Aren't the cases covered by "server-name" and ("server-name" "tool1" "tool2" ...)?

[vspinu]

@karthink

1. Yes, indeed. Only programatic usage is changed
2. It's for consistency ("server-name . nil) - no tools activated, ("server" . t) - all tools activated, ("server" 'a 'b) only tools. Such consistency is good for programatic usage, for instance a variable ("server" somevar), but it's a minor. I am fine either way. I can surely remove it if you don't like it.