feat: per-user mcp auth

i want to allow users to use mcp but to provide their own auth - so each of them work with their account on the other-party side (eg Atlassian mcp and each use own space).

1. setting to use shared or per-user auth to mcp client settings
2. allow to select unauthenticated mcp from "Tools" in prompt-input asing for auth
3. allow to `@mention` unauthenticated tool triggering auth flow