rgw/sal: list_buckets() returns RGWBucketEnts

rgw/sal: list_buckets() returns RGWBucketEnts

`sal::User::list_buckets()` no longer returns a map of `sal::Bucket`
handles. it now uses `std::span<RGWBucketEnt>` for input and output.
`RGWBucketEnt` contains all of the information we need to satisfy
ListBuckets requests, and also stores the `rgw_bucket` key for use with
`Driver::get_bucket()` where a `sal::Bucket` handle is necessary

`sal::BucketList` contains the span of results and the `next_marker`.
the `is_truncated` flag was removed in favor of `!next_marker.empty()`

the checks for `user->get_max_buckets()` on bucket creation now use a
paginated `check_user_max_buckets()` helper function that limits the
number of allocated entries to `rgw_list_buckets_max_chunk`

Signed-off-by: Casey Bodley <cbodley@redhat.com>