Add the Draft parameter for merge request structs

Currently, in GitLab 14.4.0 the `draft` url parameter is not implemented -
and I'm still unsure whether it should be? You still filter for non-draft
merge requests with `wip=yes`. If GitLab ever removes the `wip` url param
and replaces it with `draft` the typing of this field becomes a difficult
question I think, might be wrong here.

This should be backwards compatible due to the `omitempty` being in both the
`Draft` and `WIP` fields of the merge request structs.

Also adjusted the tests to use `Draft:` instead of `WIP:`. It might be worth
considering having tests for both but that would raise questions on which
behaviour to expect - the removed `WIP:` not working or until it is still in
supported versions to maintain backwards compatibility.

merge_requests.go

@@ -163,6 +163,7 @@ type ListMergeRequestsOptions struct {
 	TargetBranch           *string    `url:"target_branch,omitempty" json:"target_branch,omitempty"`
 	Search                 *string    `url:"search,omitempty" json:"search,omitempty"`
 	In                     *string    `url:"in,omitempty" json:"in,omitempty"`
+	Draft                  *bool      `url:"draft,omitempty" json:"draft,omitempty"`
 	WIP                    *string    `url:"wip,omitempty" json:"wip,omitempty"`
 }
 
@@ -219,6 +220,7 @@ type ListGroupMergeRequestsOptions struct {
 	TargetBranch           *string    `url:"target_branch,omitempty" json:"target_branch,omitempty"`
 	Search                 *string    `url:"search,omitempty" json:"search,omitempty"`
 	In                     *string    `url:"in,omitempty" json:"in,omitempty"`
+	Draft                  *bool      `url:"draft,omitempty" json:"draft,omitempty"`
 	WIP                    *string    `url:"wip,omitempty" json:"wip,omitempty"`
 }
 
@@ -278,6 +280,7 @@ type ListProjectMergeRequestsOptions struct {
 	SourceBranch           *string    `url:"source_branch,omitempty" json:"source_branch,omitempty"`
 	TargetBranch           *string    `url:"target_branch,omitempty" json:"target_branch,omitempty"`
 	Search                 *string    `url:"search,omitempty" json:"search,omitempty"`
+	Draft                  *bool      `url:"draft,omitempty" json:"draft,omitempty"`
 	WIP                    *string    `url:"wip,omitempty" json:"wip,omitempty"`
 }