Add support for Vector Stores

[OSalama]

Vector Stores are now the preferred way of attaching files to assistants, so here's support for them!

[nicolasblanco]

Yes, I would like this also 👍🏻

[EpicOrange]

Thanks for this! I'm getting this error:

Erlang/OTP 27 [erts-15.1.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

==> openai
Compiling 26 files (.ex)
      error: def vector_stores/2 defines defaults multiple times. Elixir allows defaults to be declared once per definition. Instead of:

          def foo(:first_clause, b \\ :default) do ... end
          def foo(:second_clause, b \\ :default) do ... end

      one should write:

          def foo(a, b \\ :default)
          def foo(:first_clause, b) do ... end
          def foo(:second_clause, b) do ... end

      │
 2305 │   def vector_stores(vector_store_id, config \\ %Config{})
      │       ^
      │
      └─ lib/openai.ex:2305:7