Fix incorrect handling of T61 and other strings in `mbedtls_x509_dn_gets()`

[davidhorstmann-arm]

These should be returned as strings by mbedtls_x509_dn_gets() rather than being returned in hexstring form.

Specifically:

• Change mbedtls_x509_dn_gets() to render all string types plainly rather than in OID/hexstring format.
• Add functionality to cert_write to create certificates whose subject names have a custom string tag.
• Generate test certificates that have tags of different string types.
• Add these new certificates as testcases in test_suite_x509.data.
• (Add a ChangeLog)

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [x] changelog provided
• [x] 3.6 backport #9300
• [x] 2.28 backport not required - problem doesn't exist in 2.28
• [x] tests provided

[davidhorstmann-arm]

This is ready for review, but note that it should not be allowed to clash with #9249, so it will need work (and a framework PR) once that PR is merged.

However, only the location of data files will change, so it's still worthwhile reviewing this now.