Support git ssh keys as a more secure login mechanism for updating git repos

[yagnasrinath]

Checklist

• [x] I've searched the issue queue to verify this is not a duplicate feature request.

Proposed Feature

Support git login with ssh keys

Motivation

http access tokens/password are not allowed to be used in many secure environments. ssh keys are usually allowed in such secure environments.

[krancour]

2178 is what is blocking this. I expect other changes going into v0.9.0 will incidentally address #2187, and by extension, unblock this.