Callback implementation for Plug.Parsers.init/1.

Attempts to parse the connection’s request body given the content-type type, subtype, and its parameters.

The arguments are:

• the Plug.Conn connection
• type, the content-type type (e.g., "x-sample" for the "x-sample/json" content-type)
• subtype, the content-type subtype (e.g., "json" for the "x-sample/json" content-type)
• params, the content-type parameters (e.g., %{"foo" => "bar"} for the "text/plain; foo=bar" content-type)

This function should return:

• {:ok, body_params, conn} if the parser is able to handle the given content-type; body_params should be a map
• {:next, conn} if the next parser should be invoked
• {:error, :too_large, conn} if the request goes over the given limit

Callback implementation for Plug.Parsers.parse/5.