Add API support for retrieving merge requests deployed in a deployment

In !18755 (merged) we are adding backend support for tracking merge requests deployed using GitLab environments and deployments. Said merge request does not include any APIs or UI elements to display this data. This is intentional, as it allows us to first observe how well the tracking of the data works; before investing a lot of time in UIs and APIs that may end up not working the way we want them to.

When the tracking of merge requests is deemed good enough, we should add an API that allows you to retrieve the merge requests deployed in a deployment. This API should allow filtering of merge requests like the regular merge requests API, such as filtering by labels or assignees. Care should be taken to not duplicate the entire merge requests listing API, instead we should reuse as much as possible (ideally even the supported input parameters).

The API should be supported by both the REST and Graphql APIs, though I would argue that the REST API is most important since that is probably what most (or at least we) will be using.