Allow passing Sequel connection options

[bborn]

Fixes #800 by allowing you to pass connection options to Sequel:

Langchain::Vectorsearch::Pgvector.new(
  url: ENV["POSTGRES_URL"],
  index_name: "products",
  llm: Langchain::LLM::OpenAI.new(api_key: "123"),
  connection_options: {
    max_connections: 10
  }
)