Specify custom directory for index files

[uros-r]

Further to #6:

• index_file_dir can be optionally set at collection level; otherwise defaults to current dir per existing behaviour
• a couple of tests added (tests can now use a TemporaryDirectory)

[uros-r]

As an aside, now that index filenames are derived from the collection name (f"index_{name}.hnsw"), rather than being generated dynamically, it probably makes sense to stop using filename from dbHnswIndexConfig going forward.

I've not made that change in this PR. Happy to roll it in if it does make sense.