Add connection pool to PlaywrightTest

[sax]

This is still a code sketch, as the test suite has failures such as the following:

  4) test Page network events request/response event info includes :page (Playwright.Page.NetworkTest)
     test/integration/network_test.exs:28
     ** (exit) exited in: GenServer.call(#PID<0.659.0>, {:post, {:cmd, %Playwright.Runner.Channel.Command{guid: "page@1321f1d12f28277d3410d9e74443f254", id: 271, metadata: %{}, method: "close", params: %{}}}}, 5000)
         ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
     stacktrace:
       (elixir 1.12.3) lib/gen_server.ex:1024: GenServer.call/3
       (playwright 0.1.16-preview-2) lib/playwright/page.ex:83: Playwright.Page.close/1
       (ex_unit 1.12.3) lib/ex_unit/on_exit_handler.ex:143: ExUnit.OnExitHandler.exec_callback/1
       (ex_unit 1.12.3) lib/ex_unit/on_exit_handler.ex:129: ExUnit.OnExitHandler.on_exit_runner_loop/0

I wanted to open a pull request for starting a conversation about whether this is a useful addition, and if so how runtime configuration should be used to initialize the pool. A working proof of concept would at least have a regularly passing test suite 😄

[eahanson]

If we have a connection pool, it would be nice for it to have zero startup time so that if you're running just a single test, you don't have to wait for all the connections to start up first.