feat: export fee model for the test node (#1030)

## What ❔

Export the fee model module for the era test node

## Checklist

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- [ ] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
- [ ] Code has been formatted via `zk fmt` and `zk lint`.
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- [ ] Linkcheck has been run via `zk linkcheck`.

core/lib/zksync_core/src/lib.rs

@@ -82,7 +82,7 @@ pub mod consensus;
 pub mod consistency_checker;
 pub mod eth_sender;
 pub mod eth_watch;
-mod fee_model;
+pub mod fee_model;
 pub mod gas_tracker;
 pub mod genesis;
 pub mod house_keeper;