Support custom database mapping in coordinator server

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Description

as title, sometimes, user have a table `db1.t1` in fluss, they want to map the table to `db2.t1` in paimon.

### Willingness to contribute

- [ ] I'm willing to submit a PR!

Start by locating the coordinator server entry point and any existing table-mapping configuration or handling. Clarify how a Fluss table such as db1.t1 should map to Paimon as db2.t1, then define tests that demonstrate the configured mapping and its expected behavior.

Written by the indexing model from the issue text.