Add support for aggregates, composite types, partitions, and more

[joshainglis]

This pull request adds several enhancements and new features to schemainspect.

• Support for inspecting aggregate functions. A new InspectedAggFunction class was added along with the query to retrieve aggregate function details. Aggregate functions are now included when comparing schemas.

• Support for composite types. The dependency queries were updated to properly handle composite types. Tables based on composite types are now inspected as regular tables.

• Partition tables are now handled - child partition tables are excluded from privilege and trigger inspection to avoid duplicate output.

• Schema-level and function-level privileges are now included in addition to table-level privileges. The privilege query was significantly expanded to capture all privilege types.

• Comments on all major object types (tables, views, columns, types, functions, etc) are now inspected using a new InspectedComment class and query.

• Empty string schemas are now handled properly in the quoted_full_name property.

• Various bug fixes, including:

  • Fixed issue where get_dependency_by_signature could return None
  • Fixed schema filtering in privilege query
  • Fixed an issue with negative stereotyping in the eq comparison
  • Corrected the drop statement for functions to use identity arguments

• Enabled PEP 517 building in pyproject.toml

This PR builds on top of @biodevc's fork at https://github.com/biodevc/schemainspect