[Go][client] customise location of the generated model files

Created by: gcatanese

@antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04)

The changes allow specifying in which folder the model files should be placed during the code generation. The default location can be overridden by passing the sub-directory in the additional properties ie --additional-properties=modelFileFolder=model

How to validate the changes?

Generate models passing --additional-properties=modelFileFolder=model

Fix #12307 (closed)

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