Decouple generic meta access from versioning and update ledger store and backend accordingly.
Prepares backend for additional stores with independent versions.

This refactors selected store mutations to go through nano::ledger methods instead of accessing stores directly.
This enables future maintenance of extended index tables behind an extended_ledger flag without spreading index logic across the codebase.

This change introduces an optional ext_ledger_store with its own schema, versioning, and upgrade logic.

…dicated nano::ext_ledger

Keeps data processing and index population out of the store layer.
The ext_ledger_store is now limited to schema management and version/table migrations via perform_upgrades.
This establishes a clearer separation of responsibilities between storage mechanics and ledger-level logic.

… being disabled in config

The index is automatically backfilled from existing ledger data on initialization and kept consistent via ledger put/delete block hooks.

…r thread

The index is automatically backfilled from existing ledger data on initialization and kept consistent via ledger update account hooks.

…en available

Use the account_delegators_by_weight extended ledger index to serve delegators and delegators_count RPCs when the extended ledger is available.

The index is automatically backfilled from existing pending entries on initialization and kept consistent via ledger put/delete receivable hooks.

Use the account_receivables_by_amount extended ledger index to serve receivable RPC when the extended ledger is available.