migration from old to new serialization format for storage backends

[milyin]

Describe the release item

After #1474 the stored data format for storage backends filesystem and rockdb has changed. It's probably necessary to provide migration tool for them.
To do that we need to export old deserialization without cluttering the API.
We also may expect that there will be more migrations so it maybe makes sense to provide some reusable solution.

The mirgation is needed after these updates:
eclipse-zenoh/zenoh-backend-filesystem#236
eclipse-zenoh/zenoh-backend-rocksdb#223

[milyin]

Not necessary at this moment, but prerequisites for further implementation of migration are implemented here #1523 (see discussion eclipse-zenoh/zenoh-backend-filesystem#243 (comment))