AES-GCM support

[pitbulk]

At xmlseclibs (PHP) I saw some work to provide AES-GCM . I wonder how complex is to include it as well at xmlsec.

[hoefling]

I think it shouldn't be hard; just adding another transform constant since libxmlsec already does the heavy lifting:

#if XMLSEC_VERSION_HEX > $(calc hex of 1.2.27)
    PYXMLSEC_ADD_TRANSFORM_CONSTANT(TransformAes128Gcm, "AES128_GCM");
    PYXMLSEC_ADD_TRANSFORM_CONSTANT(TransformAes192Gcm, "AES192_GCM");
    PYXMLSEC_ADD_TRANSFORM_CONSTANT(TransformAes256Gcm, "AES256_GCM");
#endif

[hoefling]

@pitbulk if you can try out the hoefling/xmlsec@aes-gcm:

pip install git+https://github.com/hoefling/xmlsec.git@aes-gcm

and confirm it working, it would be great - I will try to come up with some tests in the meanwhile.

[freekvw]

I have tried this and can confirm it works. Would be nice to get it merged :-)