Add `JsonGenerator.getPrettyPrinter()`

[cowtowncoder]

(note: offshoot of FasterXML/jackson-databind#5331)

Looks like an accessor would be needed to get reference to the active PrettyPrinter a generator instance is using (if any).

At first let's only target 3.0(.0) -- implementation must differ a bit between 2.x and 3.x anyway; so if we decide to backport (in 2.21) we can do that separately.