Introduce settings configuration for `rebase and merge with one click`
## Problem to Solve
In some cases, it may not be desirable to automatically rebase and merge a merge request. This could be because the pipelines are very old, or the commits will be rewritten and lose signatures, or any other reason a user may not want this to happen.
## Proposal
We need to introduce some new settings to make this a configuration option - either decided at the time of merge, or decided for the project entirely.
### Option 1
A new setting is introduced in the merge method that either enables or disables this behavior globally if a merge method that supports automatic rebase is being used:

### Option 2
Introduce a new section that allows more advanced configuration of the rebase options (similar to existing squash options):

issue