Add support for vector function mirror

[hermitguo]

Module installation prompt:

PostgreSQL extension 'vector' is required to enable RAG for Al agentsMore information at https://github.com/pgvector/pgvector!

Add support for vector function mirror.

thanks!

[tianon]

While this extension looks neat, it is a third party extension (not part of PostgreSQL upstream), so it will not be something we install by default.

As described on the linked page, you can find their Docker images at https://github.com/pgvector/pgvector#docker, which do pre-install their extension (and from https://github.com/pgvector/pgvector/blob/17916cad00ee580b05372768d6ff84b61442166a/Dockerfile#L5, you can see that they're FROM our images directly).