Rename ExternalCatalogFactory to FederatedCatalogFactory

[flyrain]

Rename ExternalCatalogFactory to FederatedCatalogFactory because "external catalog" is a broader concept than what this interface represents. An external catalog can be either:

1. A federated catalog — Polaris connects to a remote catalog service (Iceberg REST, Hive, Hadoop) and delegates operations to it via a ConnectionConfigInfo.
2. A notification-synced catalog — Polaris manages tables locally that are synced via the notification API, with no remote connection involved.

This factory interface only applies to the first case: it takes a ConnectionConfigInfoDpo and creates a remote catalog handle. Naming it ExternalCatalogFactory incorrectly implies
it covers all external catalog types. FederatedCatalogFactory makes the scope clear and is consistent with its existing implementations (HiveFederatedCatalogFactory,
HadoopFederatedCatalogFactory).

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 Clearly explained why the changes are needed, or linked related issues: Fixes #
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)

[jbonofre]

I also suggest to rename createExternalCatalogFactory() method in AbstractIcebergCatalogHandlerAuthzTest.

[flyrain]

Thanks @dimas-b and @jbonofre for the review. Fixed the changelog per suggestion.

I also suggest to rename createExternalCatalogFactory() method in AbstractIcebergCatalogHandlerAuthzTest.

It is using PolarisCallContextCatalogFactory, which is unrelated to FederatedCatalog

[jbonofre]

@flyrain oh yes, sorry I was confused by the similar names 😄