Add support for compressed payload

[kwach]

Hey,
we have a case where we need to limit number of bytes in the cookie. We'd implemented JWT with GZIP payload compression.
Now we'd like to use cpp-jwt to handle this cookie in some part of the system.

It is described here:
https://www.rfc-editor.org/rfc/rfc7516.html#section-4.1.3

Not much, but I could provide PR with this functionality

[arun11299]

@kwach Thanks, you can provide a PR for this issue.