Implement missing SQL statements

[sydduckworth]

Currently the mapping between ella functionality and SQL statements is incomplete.

• Topic creation
  • Expected SQL: CREATE TABLE
  • Blockers: Datafusion doesn't currently expose all the information needed to create a Topic (like index order, or custom types like tensors). We could implement partial support for creating simple topics (i.e. ones with only the default time index and only scalar fields), but a full implementation would likely require some amount of custom SQL parsing.
• Table descriptions
  • Expected SQL: DESCRIBE TABLE
  • Blockers: None
• External table creation
  • Expected SQL: CREATE EXTERNAL TABLE
  • Blockers: We need a solid plan for how external tables are handled in ella before this is a realistic option.