Add support for passing custom Finch client

[evanob]

We want to be able to configure the Finch pool used by PhoeniX.Sync to communicate with Electric in :http mode. This changes allows us to do something like below.

application.ex

  children = [
    # ...
    {Finch,
     name: MyApp.Sync.Finch,
     pools: %{
       default: [protocols: [:http1], size: 300, count: 4]
     }},
    # ...
  ]

config/prod.exs

config :phoenix_sync,
  mode: :http,
  request_opts: [finch: MyApp.Sync.Finch],
  # ...