Git Integration: support pulling from repo in pipeline

[knolleary]

Description

The first iteration of Git Integration only supports pushing to a repo in a pipeline (#5297)

The next step is to support pulling from a repo. The majority of the ground work was done in 5297 - adding pull will be a relatively smaller task.

Some things to consider:

1. The current push stage generates the file name of the snapshot based on the name of the 'source' object of the snapshot. When doing the pull, I'm not sure we can determine that in quite the same way. It may be necessary to allow the user to specify the filename.
2. The push/pull branches might want to be different to support a pr/review workflow - push to branch A, PR/review merge branch B, pull from branch B.

I suspect item 1 will be required. Item 2 can be a future iteration.

Epic/Story

No response

Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

[joepavitt]

@knolleary what's an appropriate sizing here?

[knolleary]

Sizing added.

[joepavitt]

@knolleary can we get a feature demo in #feature-demos please, just so the wider org can see it in action.