Expose `lines_added` and `lines_removed` in Merge Requests API
> Note: Issues applies to MR API in both CE and EE. Issue created in EE in anticipation of merging of project issue trackers
### Problem to solve
Determine the size of a Merge Request via the API
### Proposal
Add `lines_added` and `lines_removed` to the MR API entity.
### Permissions and Security
Existing MR authorization checks will cover this
### Documentation
API documentation updates required
### What is the type of buyer?
Core
### Links / references
issue