use deploy key to allow Federalist to clone repositories

[amirbey]

User Story

Federalist should be able to update repository data (ie: webhooks, buildStatus notifications,...) directly by using a repository deploy key and not a GitHub user token.

Background (Optional)

https://docs.github.com/en/free-pro-team@latest/rest/reference/repos#create-a-deploy-key

Acceptance Criteria

• [ ] can create and store a deploy key
• [ ] can clone the repo in the garden build using deploy key
• [ ] ~can add webhook using deploy key~
• [ ] ~can report build status using deploy key~
• [ ] Change made live via Federalist

After evaluating, edit this part:

Level of effort - <low/medium/high>

Implementation outline (if higher than "low" effort):

• [ ] <TODO>

[davemcorwin]

Proposal https://docs.google.com/document/d/1P358YP-jrsQX7JpitqVO95vEnge2Ru3sLEZZTdDM7qY/edit#