Add public getter APIs for mbedtls_x509_csr

[gabor-mezei-arm]

As part of #4683, as a result of reviewing the programs there are uses of MBEDTLS_PRIVATE fields of the mbedtls_x509_csr structure. To avoid the access of the private fields create and use new public getter APIs for the fields of the mbedtls_x509_csr structure.

Accessed fields:

• subject
• pk
• version

Affected files:

• x509/cert_write.c

[gilles-peskine-arm]

#4842 made those fields public.