Build interface to extend CLI

Similar to the idea of creating an API (part of distant-core) to integrate with the distant server, it would be handy to have an interface for extending and executing the CLI.

Reasons for this:

1. We can add something like `distant server job <...>` for use by `distant-local`. This way, it could fork a search process or (on windows) create a new process to do the searching. This avoids the need to run it internally.
2. We could move the launch logic for ssh back into ssh (maybe) by having a programmatic way to spawn a distant server without needing to provide the raw string. A builder that we can guarantee matches the CLI would be preferred.