Make jetty-http FormFields use new UrlParameterDecoder to decode with allows

[joakime]

trafficstars

Jetty version(s) 12.0.20

Enhancement Description The recently introduced UrlParameterDecoder from PR #13049 should also be used by jetty-http's FormFields class.

[joakime]

The following concerns need to be addressed to make this happen.

• The UrlParameterDecoder (which lives in jetty-util) now needs to be public (it is currently package private)
• The UrlParameterDecoder needs to be adapted to allow either a Content.Source or from a Content.Chunk (which live in jetty-io)
• Perhaps the UrlParameterDecoder.CharIterator needs to be public enough to allow jetty-http or jetty-io to house a Content.Source version of a CharIterator that the UrlParameterDecoder can use.