Problem: pebbledb is not supported as versiondb backend

pebble db support similar feature[^1] as user-defined timestamp in rocksdb, it's possible to use as a versiondb backend.

there are also user feedbacks that IOPS is much lower with pebbledb compared to rocksdb, so it sounds like a good alternative backend to try.

[^1]: https://github.com/cockroachdb/pebble/blob/dcb60b9212f9c1aae67d360191be7ff13b4e4c8b/cmd/pebble/mvcc.go

Start by tracing the existing versiondb backend implementation and its user-defined timestamp behavior, then compare it with the linked Pebble MVCC reference. Done means Pebble can be used as a versiondb backend with the similar timestamp support described in the issue, and the integration is validated.

Written by the indexing model from the issue text.