Custom multiplayer certificates

[LL2XX]

Is your feature request related to a problem? Please describe. I noticed that when Sliver is in multiplayer mode, it defaults to using a certificate named "multiplayer." I think this is a signature feature. I tried importing my self-signed certificate using the import-ca -t operator, but I found that this only affects new certificates generated with the new-operator command. The server still generates its own multiplayer certificate when multiplayer mode is enabled. This seems to be hard-coded.

Describe the solution you'd like Add a parameter to enable the use of custom certificates when enabling multiplayer mode, just like the use of custom certificates when enabling the https port