Dialect: Fix `get_pk_constraint` to return `list` instead of `set` type

[amotl]

Problem

The test suite of meltano-tap-cratedb, derived from the corresponding PostgreSQL adapter, will process database introspection return values by marshalling them to JSON. Because get_pk_constraint returns a set type, that will fail.

TypeError: Object of type set is not JSON serializable

References

This is coming from a monkeypatch to meltano-tap-cratedb.

Backlog

• [x] Software tests.
• [x] Changelog item.
• [x] Check if documentation needs to be updated.