Allow passing ParserImpl by a subclass or overwrite the events

[HeikoBoettger]

Makes it possible to overwrite the getEvent calls on the parser to add additional functionality such as filtering, modifying and inserting events. Alternatively provide a constructor to allow using subclasses of the snake yaml ParserImpl. Note: didn't replace the internal parserImpl-field since other subclasses might already use it otherwise would make sense to use the Parser-interface instead.

Why?

When using networknt/json-schema-validator with yaml files as data input format, I discovered that aliases and merges are not handled in a way that it is possible to validate the yaml files making use of this feature. While there are other discussions in the issue tracker to address this issue, I decided to manipulate the produced JsonTokens by instead of returning a VALUE_STRING producing JsonTolens as if the aliases were inlined the anchor. As it turned out it's simpler to hook into the getEvent-method and do the replacement on the snake yaml parser level. Since the code basically only invoked the getEvent-method of the parser in one place, I decided to extract a protected-method to overwrite it in my own code. This is the easiest way having to contribute a larger piece of code or having to completely copy the nextToken-method into my subclass.

Optionally I was thinking of injecting a subclass off the ParserImpl however that's also not possible, so I provide this option as well leaving other the choice how to extend the class.

[cowtowncoder]

Hmmh. Code doesn't actually... compile. I'll fix it, but going forward it'd be good to at very least build the project before PR. :)

[cowtowncoder]

@HeikoBoettger Made changes to get things to compile & the way I think you meant PR to be.

LMK if this is good; I understand the goal and adding overridability is fine. Can go in 2.18.0.

[HeikoBoettger]

@cowtowncoder Oh man, I just fired up the github web editor to get this quickly into a PR. I should have taken the time to clone the repository and checkin the actuall file I had locally. The idea with the constructor has a late idea.

Sorry for creating so much work on your side, that wasn´t my intention. Thanks for getting this ready.

[cowtowncoder]

@HeikoBoettger no problem; figured it must be something like that. And since it's your first contribution, CI didn't automatically start (to show compilation errors).