[MBEDTLS_PRIVATE] Add mbedtls_x509_crt_get_ca_istrue() accesor

[minosgalanakis]

Description

This PR adds a simple getter for the int MBEDTLS_PRIVATE(ca_istrue) member of the certificate structure.

Also adds a basic test for this accesor using existing testing certificates.

Resolves #6151

Quick link to PR specific changes -> here

PR checklist

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

• [x] changelog
• [x] backport Not required, this is MBEDTLS 3.X specific.
• [x] tests Provided