sign: Support ~/ path expansion for allowed-signers

[JacobHayes]

I'm very new to rust so not sure this is the best approach - happy to hear any suggestions. I'll add tests and a changelog entry in a bit.

Fixes #5625

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[yuja]

Thanks.

[JacobHayes]

Is it worth adding a test for this? A couple things I noticed:

• I don't see a similar test for the existing ~/ expansion in signing.key
• none of the current ssh signing tests create a UserSetting, so will need a bit of extra scaffolding since we only added the logic to .from_settings
• to avoid std::env::set_var (which seems understandably rare in the test suite) for HOME, it might be easiest to just check that allowed_signers does not start with ~/ after expansion

Happy to dig in if you think it's worth it, but figured I'd check first.

[yuja]

Is it worth adding a test for this?

I would say no. It's nice to have some e2e tests for SSH signing, but we'll need to write a fake-ssh-keygen stub or something.