campaigns: allow the target branch to be customised

[LawnGnome]

A campaign always created a changeset against the repo's default branch at present, but there are scenarios where it may be desirable to allow customising the target branch, presumably through a new, optional changesetTemplate field.

[eseliger]

Another idea would be to leverage the already existing repositories field:

repositories:
  - github.com/sourcegraph/sourcegraph
  - name: github.com/sourcegraph/src-cli
    branch: release

That way, it would also support multi-root changeset creation (monorepo support)

repositories:
  - name: github.com/sourcegraph/src-cli
    workDirs:
      - cmd
      - internal

[bobheadxi]

Feature request: let this base branch be created from the default branch if it does not yet exist

[chrispine]

see also https://github.com/sourcegraph/sourcegraph/issues/14970

[chrispine]

@eseliger Could you please add an estimate to this? Thanks!

[eseliger]

Tasks I think need to be done for this one:

• Come up with syntax and extend campaign spec schema
• Add parsing to src-cli and validate given branches, like we do with defaultBranch currently
• Make sure the reconciler takes the correct branch from the changeset spec

Overall doesn't sound too complicated and the UI already supports it, so I think the estimation of 1d should be fine.

[LawnGnome]

Feature request: let this base branch be created from the default branch if it does not yet exist

Split into #15110.