b2bua: Add AVP-based authentication

We've had the code in 30b65437170cbd0ac09acaf847b58fd7177290b1 in production for some time (with OpenSIPS 1.8.2), but I never got to submitting it.

This adds support for AVP-based authentication in the B2BUA modules; if activated this replaces `uac_auth`-based authentication.
The AVP names can be defined in the configuration of the `b2b_logic` module.

This is probably missing some steps for inclusion (documentation, style, ..), let me know.