Add GitLab Merge Request Origin

This is a presubmit origin that is able to read from a project's merge requests.

This origin doesn't handle filtering by pending state, closed state, approvals, etc correctly, but doing so would have increased the commit size larger than I would have wanted. I plan to do this in a follow-up change.

BUG=393384198
PiperOrigin-RevId: 731052478
Change-Id: I2a55743331cace6ea3d005d81a0e2ea053da21c1

Author: chris-campos

java/com/google/copybara/git/BUILD

@@ -65,7 +65,11 @@ java_library(
         "//java/com/google/copybara/exception",
         "//java/com/google/copybara/git/github:api",
         "//java/com/google/copybara/git/github:util",
+        "//java/com/google/copybara/git/gitlab:git_lab_options",
+        "//java/com/google/copybara/git/gitlab/api:gitlab_api",
+        "//java/com/google/copybara/git/gitlab/api/entities",
         "//java/com/google/copybara/go",
+        "//java/com/google/copybara/http/auth",
         "//java/com/google/copybara/jcommander:converters",
         "//java/com/google/copybara/jcommander:splitters",
         "//java/com/google/copybara/jcommander:validators",