The vector store client now supports a comma-separated list of URIs in
the `vector_store_primary_uri` configuration option.
It uses the vector store nodes from these URIs for load balancing and high
availability, querying the next node if the current one fails.
The `vector_store_client::port()` and `vector_store_client::host()` methods
were only used in the test code.
Moreover, these tests are no longer needed, as the proper parsing of the
URI is already tested in other tests that perform requests to the
vector store server mock.
This change introduces a load balancing mechanism for the vector store client.
The client can now distribute requests across multiple vector store nodes.
The distribution mechanism performs random selection of nodes for each request.
Introduce the `vs_mock_server` test class, which is capable of counting
incoming requests. This will be used in subsequent tests to verify
load balancing logic.
The `vector_store_client_test` is moved from `test/boost` to a new
`test/vector_search` directory.
This change prepares a dedicated location for all upcoming tests
related to the vector search feature.
