perforce: support 'git p4 --use-client-spec' passthrough

Created by: bobheadxi

Support passthrough of git p4's --use-client-spec option

To be frank I'm still not entirely 100% sure about everything this does after reading the p4 docs a few times, but from the docs above it seems git p4 only supports a subset of that functionality and if we support git p4... this should be fine? It's also an opt-in feature so can be used at one's own risk.

This is apparently a highly requested feature. More specifically, closes https://github.com/sourcegraph/customer/issues/445

Hopefully, someone more knowledgeable can correct me if I'm wrong! Not sure if there's any tests worth adding here either