New function mbedtls_ecp_keypair_get_group_id

Add a simple function to get the group id from a key object.

This information is available via mbedtls_ecp_export, but that function
consumes a lot of memory, which is a waste if all you need is to identify
the curve.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/ecp.c

@@ -3357,6 +3357,12 @@ cleanup:
 }
 #endif /* MBEDTLS_ECP_C */
 
+mbedtls_ecp_group_id mbedtls_ecp_keypair_get_group_id(
+    const mbedtls_ecp_keypair *key)
+{
+    return key->grp.id;
+}
+
 /*
  * Export generic key-pair parameters.
  */