Add label to changesets

[malomarrec]

Feature request

Adding codehost labels to batch changes.
This could either be a spec field, or a bulk action.

Context: we'd said no to this before because Bitbucket does not support labels.

Use cases

• Customer wants to setup a pipeline that automatically merges changesets with certain tags

[github-actions]

Hey, @sourcegraph/batchers (@eseliger @mrnugget @LawnGnome @malomarrec @chrispine @courier-new) - we have been mentioned. Let's take a look.