mon: add proxy to cache tier options

mon: add proxy to cache tier options

When we set the proxy mode to remove a writeback cache according to
the ceph official documentation an error occurred:

[root@controller-1 root]# ceph osd tier cache-mode cachepool proxy
Invalid command: proxy not in writeback|readproxy|readonly|none
osd tier cache-mode writeback|readproxy|readonly|none [--yes-i-really-mean-it]:
specify the caching mode for cache tier

According to the description of the official website document: since
a writeback cache may have modified data, you must take steps to ensure
that you do not lose any recent changes to objects in the cache before
you disable and remove it. Change the cache mode to proxy so that new and
modified objects will flush to the backing storage pool.

Fixes: https://tracker.ceph.com/issues/54576
Signed-off-by: tan changzhi <544463199@qq.com>