STIR/SHAKEN: Fix certificate type and storage.

During OpenSIPit, we found out that the public certificates must be of
type X.509. When reading in public keys, we use the corresponding X.509
functions now.

We also discovered that we needed a better naming scheme for the
certificates since certificates with the same name would cause issues
(overwriting certs, etc.). Now when we download a public certificate, we
get the serial number from it and use that as the name of the cached
certificate.

The configuration option public_key_url in stir_shaken.conf has also
been renamed to public_cert_url, which better describes what the option
is for.

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

Change-Id: Ia00b20835f5f976e3603797f2f2fb19672d8114d