mbedtls: factor out mbedtls_x509_get_subject_alt_name_ext usage

[rcmcdonald91]

Ref: #510

[Roytak]

The changes look good and are appreciated, however, there are some problems due to the library's coding style and some other small nits. I could also imagine creating a new function that parses the GeneralNames separately instead of it all being inside a single one.

[Roytak]

Could you also change the target branch from master to devel, please?

[Roytak]

The changes look good, one last thing would be to run make format, because the format test is failing. Will merge once you've run the command and committed the formatted code. Thank you.

[rcmcdonald91]

The changes look good, one last thing would be to run make format, because the format test is failing. Will merge once you've run the command and committed the formatted code. Thank you.

Done! Thanks