This commit includes the work done in collaboration with Hugo Wen from
Amazon:

    MDEV-33408 Alter HNSW graph storage and fix memory leak

    This commit changes the way HNSW graph information is stored in the
    second table. Instead of storing connections as separate records, it now
    stores neighbors for each node, leading to significant performance
    improvements and storage savings.

    Comparing with the previous approach, the insert speed is 5 times faster,
    search speed improves by 23%, and storage usage is reduced by 73%, based
    on ann-benchmark tests with random-xs-20-euclidean and
    random-s-100-euclidean datasets.

    Additionally, in previous code, vector objects were not released after
    use, resulting in excessive memory consumption (over 20GB for building
    the index with 90,000 records), preventing tests with large datasets.
    Now ensure that vectors are released appropriately during the insert and
    search functions. Note there are still some vectors that need to be
    cleaned up after search query completion. Needs to be addressed in a
    future commit.

    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.

As well as the commit:

    Introduce session variables to manage HNSW index parameters

    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

    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.
Co-authored-by: Hugo Wen <wenhug@amazon.com>