Add the possibility to allow blueprints to add custom keywords

[MathieuAA]

For instance, for the VueJS blueprint, it can add the clientFramework value vuejs

@pascalgrimaud this one deserves a bounty

[mshima]

@MathieuAA are you working on this one?
I can give a try.

[MathieuAA]

The need for this GH issue is this: I'm a blueprint dev and I don't want the JDL parsing system to throw an error if the JDL content contains any value (option value, app value, anything a blueprint can add) related to the blueprint.
A simple example so that you get the full picture: the vuejs blueprint adds the "vuejs" (or Vue, I don't remember) app value for the "client framework" option. Pierre had to add the string as a valid value back then, but this isn't a good solution as you can guess...

The issue is a possible solution, but it's not definitive. I'm working on the "add the blueprints keyword" right now, and this issue may or may not be relevant after adding the keyword.
All ideas are welcome.

[mshima]

I started to implement this one, because I had an idea on how to implement it.
The concept is done.
I will created a draft PR to you take a look.
If this is already in progress, then I will close it.

[mshima]

The blueprint jdl will be loaded like I did at jhipster/generator-jhipster#11337.

I recommend that the blueprint config should be versioned.

[MathieuAA]

Alright I'll have a look then!