Igniter setup multipart parser

[maennchen]

Is your feature request related to a problem? Please describe.

WIth #214, a parser needs to be added to the Plug.Parser config in the endpoint.

Describe the solution you'd like

Add the parser.

Describe alternatives you've considered

None

Express the feature either with a change to resource syntax, or with a change to the resource interface

plug Plug.Parsers,
  #                                         V====================V
  parsers: [:urlencoded, :multipart, :json, AshJsonApi.Plug.Parser],
  pass: ["*/*"],
  json_decoder: Jason

Additional context

Continued from #214

[zachdaniel]

This has been complete din main :)