feat(achievements): support updating the award message of a user achievement

What

Adds AwardMessage *string to UpdateUserAchievementOptions. UpdateUserAchievement sends it as awardMessage in the userAchievementsUpdate input only when set.

Tests

TestAchievementsService_UpdateUserAchievement_awardMessage verifies that the mutation input contains awardMessage and omits showOnProfile when it is not set, and that the updated message is returned.

GitLab API docs: https://docs.gitlab.com/api/graphql/reference/#mutationuserachievementsupdate