repos: add Perforce code host connection type

Created by: unknwon

This PR adds a new code host connection type for Perforce, which allows users to CRUD but not yet able to actually sync depots from the specified Perforce Server (will be on the subsequent PR).

The web UI is currently guarded by the experimental feature flag "experimentalFeatures": { "perforce": "enabled" } until the development of the RFC 277 is finished.

@sourcegraph/web: I got the web app working, but I'm sure many of parts are not idiomatic, please advise 🙇

TODO

• New schema definition
• New option in the web UI
• GraphQL endpoints
• Database CRUD
• Guarded by an experimental feature flag: Slack thread for asking help
• Replace GitIcon with PerforceIcon: Slack thread for asking how
• Tests

Fixes #16702 (closed)