Allow `upterm host` to allowlist users on self-hosted versions of VCSes

[debayande]

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Fixes #274.

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.

This PR, if merged, will enable users to use the aforementioned feature with self-hosted versions of said services as well.