Support custom auth tag length

[Andrew Berlin]

Currently A256GCM encryption method produces only 128-bit auth tags.

It would be nice to have a possibility to customise its length.

[Vladimir Dzhuvinov]

Are you aware that this would then no longer classify as A256GSM as specified in the JWA RFC?

https://tools.ietf.org/html/rfc7518#section-5.3

The requested size of the Authentication Tag output MUST be 128 bits,
regardless of the key size.

And 128 bits is the longest value taken from the NIST spec:

https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf

The bit length of the tag, denoted t, is a security parameter, as discussed in Appendix B. In general, t may be any one of the following five values: 128, 120, 112, 104, or 96.

[Vladimir Dzhuvinov]

Customising the A256GSM auth tag length is against the spec.

If there's real need for that create a custom enc identifier and content encryption method.