feat(l1): add StoreBackend impl for libmdbx

[MegaRedHand]

Motivation

Description

Checklist

• Updated STORE_SCHEMA_VERSION (crates/storage/lib.rs) if the PR includes breaking changes to the Store requiring a re-sync.

Closes #issue_number

[github-actions]

Lines of code report

Total lines added: 177
Total lines removed: 0
Total lines changed: 177

Detailed view

+------------------------------------------+-------+------+
| File                                     | Lines | Diff |
+------------------------------------------+-------+------+
| ethrex/cmd/ethrex/initializers.rs        | 457   | +2   |
+------------------------------------------+-------+------+
| ethrex/crates/storage/backend/libmdbx.rs | 161   | +161 |
+------------------------------------------+-------+------+
| ethrex/crates/storage/backend/mod.rs     | 5     | +2   |
+------------------------------------------+-------+------+
| ethrex/crates/storage/store.rs           | 2690  | +12  |
+------------------------------------------+-------+------+