REST API: Add new query attribute "draft" for merge requests instead of "wip"
## Issue
As stated in [the documentation](https://docs.gitlab.com/api/merge_requests), there is a `wip` query attribute that allows to return only no/draft merge requests. However, the response includes a `work_in_progress` attribute, which has been deprecated in favor of the new `draft` attribute. These are all different, inconsistent names:
* `wip`
* `work_in_progress`
* `draft`
Another inconsistency is that the `wip` query attribute is the only one that uses `yes`/`no` instead of `true`/`false`, making it difficult to use.
## Proposal
Add a new `draft` query attribute with `true`/`false` values that replicates the behavior of `wip`. Deprecate the `wip` query attribute.
Thus, both request and response attributes will be called `draft`, and all boolean attributes will use values of `true`/`false`.
issue