Perforce Integration: Optionally allow administrators to provide .p4trust file

Created by: mike-r-mclaughlin

Feature request description

Sourcegraph should allow administrators the option to provide their Perforce server's .p4trust file instead of using p4 trust command.

Is your feature request related to a problem? If so, please describe.

N/A

Describe alternatives you've considered.

N/A

Additional context

Customer 6716 discussion in Slack thread.