res_stir_shaken: Allow sending Identity headers for unknown TNs

res_stir_shaken: Allow sending Identity headers for unknown TNs

Added a new option "unknown_tn_attest_level" to allow Identity
headers to be sent when a callerid TN isn't explicitly configured
in stir_shaken.conf.  Since there's no TN object, a private_key_file
and public_cert_url must be configured in the attestation or profile
objects.

Since "unknown_tn_attest_level" uses the same enum as attest_level,
some of the sorcery macros had to be refactored to allow sharing
the enum and to/from string conversion functions.

Also fixed a memory leak in crypto_utils:pem_file_cb().

Resolves: #921

UserNote: You can now set the "unknown_tn_attest_level" option
in the attestation and/or profile objects in stir_shaken.conf to
enable sending Identity headers for callerid TNs not explicitly
configured.