Server: Allow spreading CAs of custom untrusted certificates via DNS TXT Records

Part B specifies that

If a SiLA Server uses an untrusted certificate, the SiLA Server MUST send a textual representation of the PEM-encoded Certificate Authority used as a TXT record. The TXT record MUST contain the certificate where each line is stored separately (given that strings in a TXT record have a limitation to 255 characters) in a string prepended with "ca=" where "l" MUST be the 0-indexed line number. This allows clients to check the certificate before attempting to connect.

This is currently done automatically for generated self-signed certificates. There should be a way for users to provide a custom untrusted certificate and have its CA spread via TXT records.