Add missing accessor for mbedtls_x509_crt.ext_types for uacme

[ndilieto]

Suggested enhancement

Add missing accessor functions to the following

• 1) mbedtls_ecp_keypair.MBEDTLS_PRIVATE(grp)
• 2) mbedtls_ecp_keypair.MBEDTLS_PRIVATE(Q)
• 3) mbedtls_x509_crt.MBEDTLS_PRIVATE(ext_types)
• 4) mbedtls_pem_context.MBEDTLS_PRIVATE(buf)
• 5) mbedtls_pem_context.MBEDTLS_PRIVATE(buflen)
• 6) SSL.MBEDTLS_PRIVATE(state)

Edit: For 1) and 2) a new mbedtls_ec_export function could be introduced, similar to mbedtls_rsa_export which already exists.

Justification

Commit ndilieto/uacme@64d1b3c adds support for mbedTLS v3.0 to uacme, but accessor functions to the above are missing and therefore I had to resort to MBEDTLS_PRIVATE.

[daverodgman]

The first two (ECP keypair) are covered by #4838. 4 & 5 (PEM context) are covered by #5414 (fix merged in #5504). 6 (SSL handshake state) is covered by #4383. So I think the only new item is relating to mbedtls_x509_crt.ext_types (I'll rename this issue for clarity).