Refactored ClarityBackingStorage trait to be more generic

[rob-stacks]

Description

The current ClarityBackingStorage trait is (still) heavily bound to sqlite. This patch makes it more generic without chnging the current logic (an additional abstraction layer for "Backing Stores" based on sqlite has been provided)

In addition to this it addresses the TrieHash/proof mechanism that right now is heavily tied to the MARF specific implementation (by allowing the proof to be invalid instead of empty)

Another patch will follow to take care of the MARF naming convention in some of the trait/structs

Note that an example/proof-of-concept/non-marf implementation (HashMapWritableStore) is provided too in tests

Applicable issues

Additional info (benefits, drawbacks, caveats)

Checklist

• Test coverage for new or modified code paths
• For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
• Changelog fragment(s) or "no changelog" label added (see changelog.d/README.md)
• Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo