Allow Printers and Parsers to be registered individually to FormatterRegistry

[philwebb]

Currently FormatterRegistry has a registerFormatter method but no registerPrinter or registerParser methods. This makes this Spring Boot pull-request a little more verbose than it should be because it's copied the private PrinterConverter and ParserConverter classes.

[sbrannen]

SGTM.

@philwebb, do you want to tackle this one yourself in time for 5.2 RC1? Or shall one of us take it over?

[philwebb]

Sorry, forgot to assign it. Working on it currently.

[renjie-rm]

Does the print method of formatter need to inject the bean itself and then call the formatting method? @philwebb

[renjie-rm]

It cannot be called through Spring like parse？

[renjie-rm]

Looking forward to your reply. Thank you！@philwebb