Add parser for decoding UTC datatypes

[bartavelle]

It requires the elm-community/json-extra package for now.

[domenkozar]

It would be great if:

• it would be imported by json-helpers so that using this library only requires adding one dependency
• it added the decoder
• would import Posix
• would use https://package.elm-lang.org/packages/rtfeldman/elm-iso8601-date-strings/latest/Iso8601 as it's more minimal

Our current workaround is:

      "import Time exposing (Posix)",
      "import Iso8601",
      "jsonDecPosix : Json.Decode.Decoder Posix",
      "jsonDecPosix = Iso8601.decoder",
      "",
      "jsonEncPosix : Posix -> Value",
      "jsonEncPosix = Iso8601.encode",