Release version of libsrtp 2 with Cryptex support?

[alvestrand]

We (Google Meet / libwebrtc) have encountered some situations where it might be nice to use Cryptex.
However, the PR for adding cryptex (#511) didn't make it into 2.7.0
Would it be possible to get a release that includes Cryptex supprot, bot doesn't require redesigning our stuff for changes coming in 3.0.0?

(libwebrtc is currently using libsrtp 2.6.0)

[pabuhler]

@alvestrand there is a patch available now, are you able to verify it will work for your usecase? Would be good before trying to merge.