Add TableConverter interface in xtable-api module

### Feature Request / Improvement

The `TableConverter` interface will be used for converting a table representation provided by the catalog (in case of Polaris it's GenericTableEntity) and convert it to the representation of the catalog's choice. The implementation of such interface can be used the catalogs through a REST service or by including it as a dependency.

https://github.com/apache/polaris/blob/main/polaris-core/src/main/java/org/apache/polaris/core/entity/table/GenericTableEntity.java

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start by reading GenericTableEntity in polaris-core/src/main/java/org/apache/polaris/core/entity/table/GenericTableEntity.java and then inspect the xtable-api module to understand the intended catalog conversion boundary. The issue does not name an implementation location, target representations, or tests; done means the agreed TableConverter contract is added to xtable-api and the module builds successfully.

Written by the indexing model from the issue text.