Support for additionalProperties in the C generator

Created by: michelealbano

I worked on the C-libcurl client. It was crashing when working on models with additionalProperties (see issue #5395 (closed)). I found 2 problems:

• the generator was creating too many nodes one into the other
• when the additionalProperties were using native datatypes (booleans, etc), it was not allocating memory for them - but was trying to "free" them

I verified the json I produce now against json produced by the python client when I use integers as additionalProperties. They both produce this JSON now: { "orchestrationFlags": { "onlyPreferred": 0, "overrideStore": 1 } }

There is still a problem with booleans, but I will solve it later.

@zhemant @ityuhui

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