Implemented generic kv-pair setting of export_config from args

[borisfom]

What does this PR do ?

This will be a basis for several export extensions that involve user input.
As an example, I have changed handling of --cache_support arg in export.py to use the new method.

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[titu1994]

Looks nice, minor changes required

[VahidooX]

LGTM! Would you please just add some documentation on how to export hybrid and cache-aware models to here:
https://docs.nvidia.com/deeplearning/nemo/user-guide/docs/en/main/core/export.html

[titu1994]

Looks great now, thanks !

[borisfom]

Superseded by #6983