tests: add PgVectorStore to integration tests

[ludwiktrammer]

PgVectorStore should be one of the tests returned by vector_store_fixture under ragbits-core/tests/integration/vector_stores/test_vector_store.py, so that the common vector store tests are run on PgVector.

Note: This task is not as trivial as it sounds, because it involves finding a way to create a testing postgreSQL instance