Add raw public key buffer bounds to mbedtls_x509_crt struct

This commit adds an ASN.1 buffer field `pk_raw` to `mbedtls_x509_crt`
which stores the bounds of the raw public key data within an X.509 CRT.

This will be useful in subsequent commits to extract the peer's public
key from its certificate chain.
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 97d4b95..e4a35f6 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -990,11 +990,13 @@
     /*
      * SubjectPublicKeyInfo
      */
+    crt->pk_raw.p = p;
     if( ( ret = mbedtls_pk_parse_subpubkey( &p, end, &crt->pk ) ) != 0 )
     {
         mbedtls_x509_crt_free( crt );
         return( ret );
     }
+    crt->pk_raw.len = p - crt->pk_raw.p;
 
     /*
      *  issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,