feature: remoted lensing

[themightychris]

Already, workstations within a team could share a dedicated HOLO_CACHE_FROM/HOLO_CACHE_TO git remote to collectively cache lens results.

A new environment variable could enable pushing lens jobs to a worker peer in the cluster to handle a build. It could potentially be implemented entirely within git infrastructure:

• A tree+commit is put together w/ source commit as a parent, efficiently describing lens to be executed and including the lens input tree.
• This commit is pushed to a special ref-space on the worker remote where a pre-receive hook executes the lens and passes lens output through the hook's output to the client
• The client displays the lens output just like it would if the lens was executed locally