Toward catalog completion

[paul-rogers]

Breaks out catalog-specific changes from the Catalog Calcite integration PR to allow easier reviewing.

• Changes catalog table entries to Druid types
• Enhanced support for external tables, including functions
• Revised catalog sync API

The key change here is to use Druid types, rather than SQL types, when declaring the schema of a table in the catalog. Druid types better match the actual Druid implementation whereas SQL types are just a SQL "view" on top of the actual Druid types. This change will allow us to use complex types which will be useful for specifying aggregate columns.

Release note

No user-visible changes yet. The docs for the catalog will appear in a later PR after the Calcite integration is complete.

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This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

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