campaigns: allow user to set git commit author

[chrispine]

This is the src-cli side of the fix for #12769. (back-end PR)

It implements the first part of RFC 229, so campaign specs can have changeset templates that now look like:

changesetTemplate:
  title: Hello World
  body: My first campaign
  branch: hello-world # Push the commit to this branch.
  commit:
    message: Append Hello World to all README.md files
    author:
      name: Chris Pine
      email: chrispine@sourcegraph.com
  published: true

Issues

• I don't know how to say "this version of src-cli goes with that version of the back-end".

Future work

• Consider: instead of just going with the default author, check for local git config and use that. (But does that behave in ways we don't like: if I create a campaign without specifying an author, then you modify the campaign spec on your machine and apply it... would it change the commit author? Would we want it to?)

[LawnGnome]

LGTM!

I think the only other thing we would need is a changelog entry noting the extra fields. Beyond that, 👍.

I don't know how to say "this version of src-cli goes with that version of the back-end".

We discussed this in our sync today, and the answer is srccli.MinimumVersion.