STIR/SHAKEN: Switch to base64 URL encoding.

STIR/SHAKEN encodes using base64 URL format. Currently, we just use
base64. New functions have been added that convert to and from base64
encoding.

The origid field should also be an UUID. This means there's no reason to
have it as an option in stir_shaken.conf, as we can simply generate one
when creating the Identity header.

https://wiki.asterisk.org/wiki/display/AST/OpenSIPit+2021

Change-Id: Icf094a2a54e87db91d6b12244c9f5ba4fc2e0b8c