Sort required properties when generating schemas

[ehennestad]

The use of set in the following piece of code results in the list of required properties being unsorted.

https://github.com/HumanBrainProject/openMINDS_generator/blob/3e701662d251a51af77e3efa804510b4728d7d34/generator/generate_json_schema.py#L112-L115

This cause the schemas to look different for each commit and it results in a lot of changed files for each commit. This really clutters the commit history. and should be fixed by sorting the required properties