Returns the configured JSON encoding library for Phoenix.

To customize the JSON library, including the following in your config/config.exs:

config :phoenix, :json_library, AlternativeJsonLibrary

Returns the :plug_init_mode that controls when plugs are initialized.

We recommend to set it to :runtime in development for compilation time improvements. It must be :compile in production (the default).

This option is passed as the :init_mode to Plug.Builder.compile/3.