[Improvement] Load certificate and key at startup

[rosahaj]

At the moment the certificate and key seem to be loaded from the disk every time a new connection is made:

ja3proxy/proxy.go

Line 89 in a485d37

cert, err := tls.LoadX509KeyPair(Config.Cert, Config.Key)

It would be more efficient to load them once at startup and keep them in memory, since users probably don't expect the proxy to recognize file changes while running.