Replace getCurrentValue()/setCurrentValue() with currentValue()/assignCurrentValue() in JsonParser/JsonGenerator

[cowtowncoder]

As part of API clean up for Jackson 3.0, we are reducing use of getter/setter style for accessing state/metadata (to replace, for example, getCurrentName() and getCurrentToken() with currentName()/currentToken()).

Similar to earlier changes, it makes sense to add currentValue() to eventually replace getCurrentValue().
Similarly add assignCurrentValue(Object) to eventually replace setCurrentValue(Object).

For 2.13 we will add the new method, make it delegate to old one at base parser/generator level: for 2.14 or later we may deprecated old method. 3.0 will only have the new method.