feat(rln-relay): allow passing in a path to the tree db

[rymnc]

Problem

With the integration of pmtree into zerokit and the downstream to nwaku, we should now accept a path to the tree db to avoid onchain resyncs.

Suggested solution

• A new cmd line arg to nwaku --rln-relay-tree-db which accepts an absolute path to the tree db.
• Validation to ensure that the same db config is used (already handled on the zerokit end)
• If the db doesn't exist at the given path, we need to persist the db there
• Set metadata like LAST_BLOCK_PROCESSED into the tree upon each block being processed so that nwaku knows where to resync from.

Alternatives considered

Additional context

Add any other context or screenshots about the feature request here.

Acceptance criteria

• Operators are able to pass in the path to an existing db