Add git plugin

[plopoyop]

Summary

Implement Git plugin support for loading plugins from any Git repository with revision/branch/directory targeting. Extends existing GitHub plugin functionality to support self-hosted and local Git repositories. Fix #2048 in a more simple way than #2470 since there is no change in json schema

How was it tested?

Manually on a mock project with multiple transport : git+ssh, git+http, git+file

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers under the terms of the Apache 2 License.

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