cls/rgw: restore backward-compatible encoding of cls_rgw_bucket_instance_entry

cls/rgw: restore backward-compatible encoding of cls_rgw_bucket_instance_entry

the encoding of `cls_rgw_bucket_instance_entry` changed in
bd0b0e5d984c83504f88fa155a7dfc6f6571aa49 such that pacific/quincy
OSDs cannot read the version written by reef OSDs

two fields (bucket_instance_id and num_shards) were removed in the v2
encoder, but the v1 decoder still expects them to be present

since we haven't released reef yet, we can fix this with a v3 that adds
the fields back

Fixes: https://tracker.ceph.com/issues/57232
Signed-off-by: Casey Bodley <cbodley@redhat.com>