Python: Add vector search to Postgres connector

[lossyrob]

Motivation and Context

Following up on #8951, this PR adds an implementation of VectorSearchBase to PostgresCollection. This implementation provides vectorized search and does not implement text search or vectorizable text search.

Unit and integration tests are added, and the python/samples/getting_started/third_party/postgres-memory.ipynb notebook was expanded to include vector search in the example.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
TOTAL	16695	1778	89%

report-only-changed-files is enabled. No files were changed during this commit :)

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3005	4 💤	0 ❌	0 🔥	1m 13s ⏱️