cls/rgw_gc/cls_rgw_gc: read config via cls_get_config

cls/rgw_gc/cls_rgw_gc: read config via cls_get_config

Commit https://github.com/ceph/ceph/commit/3877c1e37f2fa4e1574b57f05132288f210835a7
added new way to let CLS gain access to global configuration (`g_ceph_context`).

`cls_rgw_gc_queue_init` method is not using the new CLS call of `cls_get_config`
but instead directly uses `g_ceph_context`.

Crimson OSD implementation does **not** support `g_ceph_context` which results in a (SIGSEGV)
crash due to null access. Switching to `cls_get_config`, similarly to `cls_rgw.cc`, would allow
both OSD implementations to access the conf safely.

The above approach is well-defined due to the two orthogonal implementations of objclass.cc.
Classical OSD uses `src/osd/objclass.cc` While Crimson OSD uses `src/crimson/osd/objclass.cc`.

Signed-off-by: Kautilya Tripathi <kautilya.tripathi@ibm.com>