Configure signature algorithms via config file

It would be useful to configure signature algorithms via config file similar to TLS DH groups. As the DH groups, it shall also be possible to specify the desired order via config file.

maybe use: auth_cfg.h -> add_pubkey_constrains
- see [Swanctlconf](https://wiki.strongswan.org/projects/strongswan/wiki/Swanctlconf] in local.auth

ryru / strongswan

Configure signature algorithms via config file #27