vsb.py: New entrypoint for benchmarking

Create vsb.py which is the main program to run vector search
benchmarks using the locust framework. Uses locust as a library,
allowins us more control over how it runs and with a much simpler set
of arguments, which at present consist of:

options:
  -h, --help            show this help message and exit
  --database {pinecone,pgvector}
                        The vector search database to test
  --workload {mnist,mnist-test}
                        The workload to run
  --clients CLIENTS     Number of clients concurrently accessing the database
  --api_key API_KEY
  --index_name INDEX_NAME