Add creating an introducer from a files

- Introducers can now be created by specifying a file. The certificate
  gets imported and declared as an introducer for the passed domains.
- Add some checks, so that the imported certificate can actually be used
  as an introducer (certificate is alive, not revoked, has certification
  capabilities)

src/commands.rs

@@ -16,6 +16,10 @@ pub enum CommandError {
     CertNotFound(Fingerprint),
     #[error("Certificate not usable: {0}")]
     CertNotUsable(Fingerprint),
+    #[error("Certificate {0} is not alive (is expired or created in the future)")]
+    CertNotAlive(Fingerprint),
+    #[error("Certificate is revoked: {0}")]
+    CertRevoked(Fingerprint),
 }
 
 pub async fn dispatch(cli: CliArgs, config: HuskConfigContainer) -> Result<()> {