feat: add support to pgvector

[mhordynski]

Feature description

The goal of this issue is to add a new VectorStore that utilizes pgvector, an extension for PostgreSQL, to store its vectors.

By default, the metadata for the entries should be stored alongside the vectors in the same PostgreSQL table.

Motivation

Many applications opt for pgvector because they are already using PostgreSQL as their relational database. Additionally, pgvector is offered as a managed database by all major cloud providers.

Additional context

No response