vectorstores: Added the option to specify a HNSW index for improving performance

[pieterclaerhout]

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
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• Contains test coverage for new functions.
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You can read more about this type of index and why you would like to use it on:

https://github.com/pgvector/pgvector#hnsw

[tmc]

Lovely. As we're adding more functionality here we probably should start using go-sqlmock to assert more about the behavior here.