Support pgvector

pgvector : https://github.com/pgvector/pgvector is open-source vector similarity search for Postgres

which means you should implement a vectorstore in langchaingo https://github.com/tmc/langchaingo/blob/main/vectorstores/vectorstores.go#L12 

You can host code under `kubeagi/arcadia pkg/vectorstores/pgvector`

