rgw/d3n: store cached objects using hash digest

rgw/d3n: store cached objects using hash digest

storing S3 objects under deep directory hierarchy resulted in a long
path which iafter URL encoding resulted in a long cached filename that
is longer than the POSIX max of 256 chars manifesting as writes to D3N
cache failing with:

```
❯ errno 36
ENAMETOOLONG 36 File name too long
```

Fixes: https://tracker.ceph.com/issues/70333
Signed-off-by: Mark Kogan <mkogan@redhat.com>
(cherry picked from commit 5bfd0b7100d32bcf793c26b007b1403db8e0a42f)