[strongSwan-dev] WI: key_encoding to cred_encoding migration

[Martin Willi]

Work-Item: key_encoding to cred_encoding migration

Branch: master

Schedule: 4.4.1

Rationale:
The pki tool in 4.4.0 supports encoding of keys into the PEM format, but
lacks the same for other credentials, such as certificates. The reason
for this is that the complete encoding/fingerprinting framework is
designed for keys only, but not for other credentials. To support
different encoding methods with the same framework, we should make it
more generic. The certificate implementations then can use the framework
to encode themselves to other formats.

API changes:
The key_encoding_t class and associated constants have been moved from
credentials/keys/key_encoding.[ch] to credentials/cred_encoding.[ch].
The constants used for the encoding process have been streamlined.
The certificate_t.get_encoding() signature has been extended, it now
accepts an encoding type and returns a boolean success value.