Make it easier via command line to add a spfs proxy repo

[jrray]

For our render farm jobs we use a job-based proxy repo that delegates to our normal "origin" repo. Setting this up is fairly tedious:

• Configure the "jobrepo" repo: SPFS_REMOTE_jobrepo_ADDRESS=file:/path/to/job/repo?create=true
• Configure an alias for the normal "origin" repo: SPFS_REMOTE_remoteorigin_ADDRESS=file:/path/to/origin/repo
• Override the "origin" repo to combine the above as a proxy repo: SPFS_REMOTE_origin_ADDRESS=proxy:?primary=jobrepo&secondary[0]=remoteorigin

(Is there already an easier way to accomplish this?)

This pattern of inserting a different repo in front of the normal "origin" repo in a proxy repo could be made easier if we had a repo argument for it like:

spfs run --insert-proxy-repo /path/to/job/repo <ref>

or

spfs run --insert-proxy-repo-to origin:/path/to/job/repo <ref>

Names and usage t.b.d, but the intention is to make it possible to just name the path to inject, and maybe the name of the repo that is to be combined with the path, if wanting to do this with something besides the "origin" repo.

Currently, spfs run doesn't take any repo arguments like -r, so AFAIK the only way to set up this kind of proxy repo configuration is to use the environment variables shown above.