Add support to read and write certificates with chain certificates

[mrtrev]

This will allow the libstirshaken library to read and write files containing the chain certificates required by some certificates in order to pass validation. The new functionality has been implemented as such that existing applications will not break and may test for support of these new functions.

These changes are confirmed working in a production environment.

[signalwire-ci[bot]]

Unit-tests compilation failed: https://public-artifacts.signalwire.cloud/drone/signalwire/libstirshaken/20/unit-tests-build-result.txt

[signalwire-ci[bot]]

Scan-build compilation failed: https://public-artifacts.signalwire.cloud/drone/signalwire/libstirshaken/20/scan-build-result.txt

[signalwire-ci[bot]]

Unit-tests failed: https://public-artifacts.signalwire.cloud/drone/signalwire/libstirshaken/28/artifacts.html

[andywolk]

Unit-tests failed: https://public-artifacts.signalwire.cloud/drone/signalwire/libstirshaken/28/artifacts.html

This PR contains changes from another PR. Please use the branch of the other PR here. Not master.

[mrtrev]

Sorry I'm not used to the github workflow. I'm not sure how to remedy this... I don't think I can change the branch the PR is from. Should I revert the change and force push this branch or do I need to create a whole new PR from a different branch and close this one?

[mrtrev]

Ok I think it is sorted out and back to a coherent PR.

[andywolk]

@mrtrev Let's not create huge commits changing spacing in the whole codebase. Please remove it and fix what we touch only.

[andywolk]

Ok. Looks good to me. @mrtrev Does it still work? And let's merge if so.

[mrtrev]

It still compiles. I'll put it in prod and will advise tomorrow if nothing abnormal observed.

[mrtrev]

@andywolk Confirmed it is reading & writing certificates correctly