rbd-mirror: integration of the new GroupSnapshotNamespaceMirror::complete field

rbd-mirror: integration of the new GroupSnapshotNamespaceMirror::complete field

This commit introduces the new field **complete**, of type **MirrorGroupSnapshotCompleteState** enum,
to the GroupSnapshotNamespaceMirror structure. This change is necessary to align behavior of
mirror group snapshots with that of mirror image snapshots, allowing for a precise differentiation
between a group snapshot that has been created and one that has been fully synced.

**1. Handling Old-Style Snapshots**

Decoding Old Snapshots: The original GroupSnapshotNamespaceMirror structure lacked the complete field,
which implicitly defaulted to a bool value of false upon initialization.
When an old snapshot (lacking the complete field) is decoded by an upgraded client,
the implicit default value maps to MIRROR_GROUP_SNAPSHOT_COMPLETE_IF_CREATED.

Completion Check: A snapshot is determined old by checking it's complete filed i.e
complete == MIRROR_GROUP_SNAPSHOT_COMPLETE_IF_CREATED and if it's old the sync completion
for these group snapshots is determined by checking the state field
i.e state == GROUP_SNAPSHOT_STATE_CREATED.

During a upgrade where **OSDs have not yet been updated**, the new client will be forced to create
snapshots using the old style. These snapshots will be initialized with MIRROR_GROUP_SNAPSHOT_COMPLETE_IF_CREATED
and will stay on that to prevent immediate, incorrect cleanup by the old OSDs and in this case
state field is set to **GROUP_SNAPSHOT_STATE_CREATED** only after snapshot completed it's sync.

**2. Handling New-Style Snapshots**

New snapshots are initialized with complete == **MIRROR_GROUP_SNAPSHOT_INCOMPLETE**,
state == GROUP_SNAPSHOT_STATE_CREATING. The group snapshot's state is marked as GROUP_SNAPSHOT_STATE_CREATED
as soon as its metadata is fully available and stored.

Completion Check: The snapshot's sync is confirmed only when complete == MIRROR_GROUP_SNAPSHOT_COMPLETE
along with state check (state == GROUP_SNAPSHOT_STATE_CREATED) is satisfied.

This approach ensures seamless transition and compatibility, allowing the system to correctly interpret the
synchronization status of both old and newly created group snapshots.

Signed-off-by: VinayBhaskar-V <vvarada@redhat.com>
Resolves: rhbz#2396583