Add ability to specify Postgres connection credentials

[michaelbautin]

Allow specifying PostgreSQL credentials when running against an external instance of PostgreSQL not managed by the benchmark. Also allow avoiding an attempt to start PostgreSQL service, because it might be managed in a different way.

Fixes: #563

[maumueller]

Thanks for your contribution, @michaelbautin. I'm a bit skeptical about the proposed solution, because it makes this very pronounced from an API point of view, while it only applies to a very small subset of the algorithms.

Can you achieve the same functionality by specifying these credentials in the config file https://github.com/erikbern/ann-benchmarks/blob/main/ann_benchmarks/algorithms/pgvector/config.yml? If I remember correctly, you can provide it in the base_args entry which is propagated to the constructor of the class.

[michaelbautin]

@maumueller Thank you for your feedback. I've reworked the PR to avoid introducing generic command-line parameters, and instead made it controllable by Postgres-specific environment variables. Also I've added automatic pgvector extension creation and a way to avoid attempting to start PostgreSQL service if it is managed in a different way.

Cc @erikbern