Support returning Pager for getMergeRequest inside MilestonesApi

[brochard]

Hi, for my current project I try to retrieve all merge requests of a Milestone to build a Release note but I'm currently limited to the default page size (96) with this method : https://github.com/gitlab4j/gitlab4j-api/blob/main/src/main/java/org/gitlab4j/api/MilestonesApi.java#L498
I think it would make sense to be able to return a pager like pretty much every other methods.

I will be attempting to implement it if that's ok with you.

Update : attempted to fix this in pull request #1103

[jmini]

I will check the PR