add benchmark scripts

[sivukhin]

Context

Several benchmark tools which I use for vector feature. This PR includes following scripts:

• benchtest - generic tool which measure performance of any sequence of SQL statements
• anntest - tool for measuring n-recall@k of ANN vector search
• blobtest - simple tool which aims to prove that sqlite3_blob_reopen API can result in way better performance

See README.md for more details

[penberg]

Nice, @sivukhin!

[haaawk]

In anntest you call srand(time(NULL)); at the beginning of main but you don't do that in the other two tests. Is this on purpose?

[sivukhin]

@haaawk - I don't need srand in the anntest actually. This is "legacy" - initially I generated ANN workload right inside anntest C code but later decided to move it to the python workload.py script as it's easier to generate random floats with different distributions from there.

Removed it from the branch