[11.11] Add `expires_at` parameter to `Groups::addMember()`

[ajuchacko]

Fix: #738

• [x] Updated test ShouldaddMember test
• [x] Moved/Made ACCESS_LEVELS const for project/group access levels
• [x] Updated phpdoc
• [x] All tests passing local

Hey, @GrahamCampbell I've added an optional parameter(of array type) to addMember method, I'd like to make another pr removing both $user_id, $acess_level from the method parameters, instead they will be passed inside the array like we're doing in other places to branch 12.0 once this gets merged

Thanks 😀

[GrahamCampbell]

Thanks.