Implement ClientKeyExchange writing in PSA-based ECDHE suites

- Populate the ECDH private key slot with a fresh private EC key
  designated for the correct algorithm.
- Export the public part of the ECDH private key from PSA and
  reformat it to suite the format of the ClientKeyExchange message.
- Perform the PSA-based ECDH key agreement and store the result
  as the premaster secret for the connection.

library/ssl_tls.c

@@ -9367,6 +9367,11 @@ void mbedtls_ssl_handshake_free( mbedtls_ssl_context *ssl )
     ssl_buffering_free( ssl );
 #endif
 
+#if defined(MBEDTLS_ECDH_C) &&                  \
+    defined(MBEDTLS_USE_PSA_CRYPTO)
+    psa_destroy_key( handshake->ecdh_psa_privkey );
+#endif /* MBEDTLS_ECDH_C && MBEDTLS_USE_PSA_CRYPTO */
+
     mbedtls_platform_zeroize( handshake,
                               sizeof( mbedtls_ssl_handshake_params ) );
 }