[typescript-axios] Add stringEnums option

[ty-v1]

This PR adds stringEnums option to typescript-axios generator.
Closes #10943

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  ./bin/utils/export_docs_generators.sh
  

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[macjohnny]

Thanks for your contribution!

[macjohnny]

@ty-v1 can you update the samples please? you need to add samples/client/petstore/typescript-axios/builds/with-string-enums/, see https://github.com/OpenAPITools/openapi-generator/runs/4919491575?check_suite_focus=true

[ty-v1]

Sorry, I could not work on it because I bought a new PC.
Now that the setup is complete, I'll update the samples.

[macjohnny]

LGTM

[kasperpeulen]

Would be great to have this for typescript-fetch as well @ty-v1