Add "View on Perforce" for Perforce code

Created by: mike-r-mclaughlin

Requested by: https://github.com/sourcegraph/accounts/issues/6716

Feature request description

Add a button/link on the file viewer that allows users to open the code in Perforce Swarm similar to the "Open in GitHub" button.

Additional details in this internal Slack thread

Describe alternatives you've considered.

Write a custom extension

/cc @muratsu @jjinnii @ryankscott