[for fleet provisioning with CSR to support write key and certificate to disk] support optional write generated private key

[giuspen]

Support optional write generated private key to disk

Description

For use by the fleet provisioning with CSR demo ( see https://github.com/giuspen/aws-iot-device-sdk-embedded-C/tree/GP_fleet_provisioning_with_csr_demo/demos/fleet_provisioning/fleet_provisioning_with_csr ), I would like to support writing key and certificate to disk when the optional macros DOWNLOADED_CERT_WRITE_PATH and GENERATED_PRIVATE_KEY_WRITE_PATH are provided

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fd53f1b) 94.02% compared to head (a4d1f61) 94.02%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #183   +/-   ##
=======================================
  Coverage   94.02%   94.02%           
=======================================
  Files           3        3           
  Lines        1689     1689           
  Branches      442      442           
=======================================
  Hits         1588     1588           
  Misses         17       17           
  Partials       84       84           

Flag	Coverage Δ
unit_tests	94.02% <ø> (ø)

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Files	Coverage Δ
source/portable/mbedtls/core_pkcs11_mbedtls.c	93.93% <ø> (ø)

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[Skptak]

Hey, thanks for submitting all the PRs you've raised to add in this feature!
I've commented two smaller nits that I saw in your changes on this PR.

I do like this idea, as I can see how useful it is for debugging. That said, I think using this feature in the file should generate a #warning just to make sure that it isn't accidentally left in.

[giuspen]

Thanks @Skptak for the review I'll also add the compilation warning shortly

[giuspen]

/bot run formatting