add halfvec indexing and queries to periodic pgvector performance tests

[Bodobolero]

Problem

halfvec data type was introduced in pgvector 0.7.0 and is popular because
it allows smaller vectors, smaller indexes and potentially better performance.

So far we have not tested halfvec in our periodic performance tests.
This PR adds halfvec indexing and halfvec queries to the test.

[github-actions]

3216 tests run: 3074 passed, 0 failed, 142 skipped (full report)

---

Flaky tests (2)

Postgres 16

• test_subscriber_restart: debug

Postgres 14

• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 31.5% (6644 of 21063 functions)
• lines: 48.6% (51674 of 106296 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
6443145 at 2024-06-14T15:51:22.886Z :recycle:}

[Bodobolero]

new test run triggered here https://github.com/neondatabase/neon/actions/runs/9516115370

[Bodobolero]

also adjusted grafana dashboards showing the metrics for pgvector indexing and queries