remote_store_address: support unix sockets

[aviau]

Hello!

As suggested in the pants docs, I use bazel-remote for caching results.

Bazel-remote support listening to unix sockets like so:

• --grpc_address=unix+grpc://path.sock

Would it be possible to support to connect to such a socket with Pants?

Currently, I face the following error:

OptionsError: The `[GLOBAL].remote_store_address` option has invalid value `unix://test.sock`: it does not have a supported scheme.

The value must start with one of: `grpc://`, `grpcs://`, `experimental:file://`, `experimental:github-actions-cache+http://`, `experimental:github-actions-cache+https://`

A scheme such as unix+grpc://path.sock would make sense.

Cheers,