Stir/Shaken Refactor

Stir/Shaken Refactor

Why do we need a refactor?

The original stir/shaken implementation was started over 3 years ago
when little was understood about practical implementation.  The
result was an implementation that wouldn't actually interoperate
with any other stir-shaken implementations.

There were also a number of stir-shaken features and RFC
requirements that were never implemented such as TNAuthList
certificate validation, sending Reason headers in SIP responses
when verification failed but we wished to continue the call, and
the ability to send Media Key(mky) grants in the Identity header
when the call involved DTLS.

Finally, there were some performance concerns around outgoing
calls and selection of the correct certificate and private key.
The configuration was keyed by an arbitrary name which meant that
for every outgoing call, we had to scan the entire list of
configured TNs to find the correct cert to use.  With only a few
TNs configured, this wasn't an issue but if you have a thousand,
it could be.

What's changed?

* Configuration objects have been refactored to be clearer about
  their uses and to fix issues.
    * The "general" object was renamed to "verification" since it
      contains parameters specific to the incoming verification
      process.  It also never handled ca_path and crl_path
      correctly.
    * A new "attestation" object was added that controls the
      outgoing attestation process.  It sets default certificates,
      keys, etc.
    * The "certificate" object was renamed to "tn" and had it's key
      change to telephone number since outgoing call attestation
      needs to look up certificates by telephone number.
    * The "profile" object had more parameters added to it that can
      override default parameters specified in the "attestation"
      and "verification" objects.
    * The "store" object was removed altogther as it was never
      implemented.

* We now use libjwt to create outgoing Identity headers and to
  parse and validate signatures on incoming Identiy headers.  Our
  previous custom implementation was much of the source of the
  interoperability issues.

* General code cleanup and refactor.
    * Moved things to better places.
    * Separated some of the complex functions to smaller ones.
    * Using context objects rather than passing tons of parameters
      in function calls.
    * Removed some complexity and unneeded encapsuation from the
      config objects.

Resolves: #351
Resolves: #46

UserNote: Asterisk's stir-shaken feature has been refactored to
correct interoperability, RFC compliance, and performance issues.
See https://docs.asterisk.org/Deployment/STIR-SHAKEN for more
information.

UpgradeNote: The stir-shaken refactor is a breaking change but since
it's not working now we don't think it matters. The
stir_shaken.conf file has changed significantly which means that
existing ones WILL need to be changed.  The stir_shaken.conf.sample
file in configs/samples/ has quite a bit more information.  This is
also an ABI breaking change since some of the existing objects
needed to be changed or removed, and new ones added.  Additionally,
if res_stir_shaken is enabled in menuselect, you'll need to either
have the development package for libjwt v1.15.3 installed or use
the --with-libjwt-bundled option with ./configure.