Configurable pull/merge requests

### Describe the problem

Currently, Weblate creates merge (pull) requests with provided text, but offers no further customization.

What might be useful to set as well:

* Assignee of the pull request (text field for username)
* Assign to acting user, if available (checkbox)
* Automatic merging (dropdown, selecting merge style)
* Add labels

Both will require additional privileges for Weblate on the repository – a regular contributor cannot change properties of the request.

### Describe the solution you'd like

Additional fields in the component configuration to customize VCS behavior. This is pretty similar to https://github.com/WeblateOrg/weblate/issues/1897 plans for the file formats.

### Describe alternatives you've considered

An alternative approach could be an add-on to adjust this. This might be a good fit as well, as turning this on will require granting Weblate more privileges on the repository as well.

### Screenshots

_No response_

### Additional context

_No response_