Add per-database configuration parameters

[daverigby]

Different databases require different configuration parameters, of different types:

• Mandatory parameters - e.g. username/password for authentication or api_key, hostname or index name to identify index
• Optional / variable parameters - e.g. pgvector's HNSW index takes M and ef_construction to control index speed vs recall tradeoff.

We need to pass these options through when that database is selected, in a scalable manner (we don't really want to show the union of all options across all databases). We also want to set sensible default values for them - e.g. pgvector should default to settings to connect to the local Docker container.

This also has some impact in how results are collected and reported - if we want to perform benchmark runs with a range of values for ef_construction for example, how should those results be reported?

[daverigby]

Pydantic has a related package - https://github.com/pydantic/pydantic-settings - which handles much of this and integrates with Pydantic's models (which we already use in VSB). However it doesn't have support for overriding on the command line, which is a significant omission IMO.

There is an open Issue and linked PR to add command-line support - see pydantic/pydantic-settings#209 - but as of writing is not yet merged...

[daverigby]

This has mostly been implemented via #82 - each database has it's own arguments in an option group, and when specifying a given database, the arguments required for it are checked.