Allow `upterm host` to authorise users on self-hosted versions of supported version control services

[debayande]

Hi.

upterm host currently supports the --{codeberg, github, gitlab, srht}-user options to allow specified users on Codeberg, GitHub, GitLab and SourceHut respectively to connect by allowlisting their public keys. This feature, however, is supported only on cloud-hosted versions of said services.

I'd like to request for upterm host to be extended to support self-hosted versions of said services as well.

Thank you.