MDEV-33408 Introduce session variables to manage HNSW index parameters

[HugoWenTD]

Description

This pull request introduces three session variables to manage HNSW index parameters, as suggested by Vicentiu in this comment

Three variables:

hnsw_max_connection_per_layer
hnsw_ef_constructor
hnsw_ef_search

ann-benchmark tool is also updated to support these variables in commit https://github.com/HugoWenTD/ann-benchmarks/commit/e09784e for branch https://github.com/HugoWenTD/ann-benchmarks/tree/mariadb-configurable

How can this PR be tested?

Execute support-files/ann-benchmark/run-local.sh, and clean up local ann-workspace/ann-benchmarks/ if there was one.

By default the following combination defined in config.yaml will be tested.

Found cached result
  0:                                  MariaDB(m=16, ef_construction=64, ef_search=40)        0.918      516.967
Found cached result
  1:                                  MariaDB(m=50, ef_construction=10, ef_search=10)        0.989      967.644

Modifying the local config.yaml file for more combinations.

---

All new code of the whole pull request, including one or several files that are either new files or modified ones, are contributed under the BSD-new license. I am contributing on behalf of my employer Amazon Web Services, Inc.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 3 committers have signed the CLA.

:white_check_mark: cvicentiu
:x: vuvova
:x: HugoWenTD
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[vuvova]

it's included in (squashed with) e317023