Add support for :otp_app option in ConnGRPC.Pool for configuration loading

[yordis]

Hey team, thank you so much for package, we are trying to adopt and we found that we needed to call Application.get_env/2 in the application.

This PR adds support for loading pool configuration from Application.get_env/3 when using the module-based pool pattern. Added the :otp_app option to the __using__ macro, allowing pools to load their configuration from application environment:

defmodule MyApp.MyPool do
  use ConnGRPC.Pool, otp_app: :my_app
end

Configuration can then be set in runtime.exs:

config :my_app, MyApp.MyPool,
  pool_size: 5,
  channel: [address: System.get_env("MY_APP_MY_POOL_ADDRESS"]

Useful to configure things from runtime.ex without the developers having to come up with that many ways to configure the pools