Support SSH/SCP protocol for mirrors and build caches

[sethrj]

Allow mirroring (both source and build caches) from remote servers over SSH.

Rationale

We have several institutional clusters on which we install approximately daily development updates of our export-controlled code for user testing. I'd like to improve deployment time and assuring quality by building with Spack as part of the CI system, then distributing that tested binary directly to the other servers.

Description

Ideally a remote server could be added with ssh:// or scp:// as opposed to the currently supported https://, file://, s3:// etc.

Additional information

See https://github.com/spack/spack/issues/6937 asking whether SSH is currently implemented.

[rbberger]

I have similar use cases and I started looking into adding that feature.