Support HTTPS for Toxiproxy API server

[miry]

Currently API could be accessed only via HTTP.
Make HTTPS would simplify some extra work to run extra reverse proxy with HTTPS support.

Possible example of set HTTPS config:

toxiproxy --cert <bundle certificate file> --cert-key <private key> --listen https://0.0.0.0:8083

[neufeldtech]

Some Questions for consideration:

• Should we allow configuring multiple certificates via file(s) and then allow the user to assign these certificates to certain proxies (or the Admin API)
• I can see use-cases where users might want only some of their proxy listeners to respond via TLS and not all, and they may need separate certificates as well