repospec: support ssh urls with ssh certificates

[KnVerey]

Closes #4741

With all the work we've put into RepoSpec lately, this is now much easier to do.

Copying that PR's description:

Organizations that use a custom SSH Certificate Authority with GitHub
Enterprise Cloud will have a host that starts with org-12345@ instead
of git@. This removes the hard-coded git@ in the repo spec parsing
for a regexp match on a valid username instead.

E.g. now it will be able to parse specs like org-12345@github.com:someOrg/someRepo/README.md?ref=someBranch.

Docs for the SSH Certificate feature are here: https://docs.github.com/en/enterprise-cloud@latest/organizations/managing-git-access-to-your-organizations-repositories/about-ssh-certificate-authorities#about-ssh-urls-with-ssh-certificates

We can see in the git documentation that the username segment is universally valid for the ssh protocol and the scp-like format: https://git-scm.com/docs/git-fetch#_git_urls. We could validate against it being used with other protocols, but falling back on the leave-validation-to-git stance, I'm inclined not to. (Yes, this is a reversal of my stance on the original PR. I've learned a lot about RepoSpec in the interim!)

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[KnVerey]

FYI @mightyguava

[natasha41575]

/lgtm

[mightyguava]

Thank you @KnVerey for getting this through! I really appreciate it. This'll be really helpful to us.