Extract IcebergCatalog into a standalone reusable crate

[Skeletrox]

Follow-up from PR #37 review:

1. Catalog persistence: Ensure the catalog can read from a saved state and come back up after a restart.
2. Separate repository: Explore moving the IcebergCatalog trait and its implementations into a standalone crate/repository so it can be shared as a dependency across other projects in this space.

Refs: #36