Draft: allow clients to overwrite default vpn cert and key type

allow clients to overwrite default vpn cert and key type when fetching from /5/openvpn/cert by appending an algo parameter with the allowed values 'rsa', 'ecdsa' and 'ed25519'.

this allows clients to tell menshen which key types it is able to process. Workaround for Android currently, which doesn't accept ed25519 for whatever reason (even wit the fixed private key encoding done in !61 (closed))