Adding new functionality

[aleks-programmer]

Please add functionality to find merge requests by author_id and other parameters.
It would be great to find by parameters because in GitLab API it is implemented
https://docs.gitlab.com/ee/api/merge_requests.html#list-merge-requests

[mitarcon]

@gmessner I can help with this. Do you have any problem?

[gmessner]

@mitarcon
I just have not had time to implement this. If you would like to implement and submit a PR, I would be happy to review it.

[gmessner]

@aleks-programmer @mitarcon
Support for filtering merge requests has been added. Use this functionality, as follows:

MergeRequestFilter filter = new MergeRequestFilter()
    .withAuthorId(anAuthorId)
    .withTargetBranch("master");
Pager<MergeRequest> pager =  gitLabApi.getMergeRequestApi().getMergeRequests(filter, 20);

All of the possible filter parameters are supported, with fluent methods. See the MergeRequestFilter class for the available withXYZ() methods.