res_stir_shaken: Add STIR_SHAKEN_ATTESTATION dialplan function.

res_stir_shaken: Add STIR_SHAKEN_ATTESTATION dialplan function.

Also...

* Refactored the verification datastore process so instead of having
a separate channel datastore for each verification result, there's only
one channel datastore with a vector of results.

* Refactored some log messages to include channel name and removed
some that would be redundant if a memory allocation failed.

Resolves: #781

UserNote: The STIR_SHAKEN_ATTESTATION dialplan function has been added
which will allow suppressing attestation on a call-by-call basis
regardless of the profile attached to the outgoing endpoint.