Add TLS functionality to socket_server

[monban]

I have a need to ingest encrypted rsyslog messages (syslog over TLS). The easiest solution seemed to be to add this capability to the SocketServer component. I have not contributed to this project before and would like feedback on my implementation. Thank you.

[Jeffail]

Hey @monban, just a quick heads up it looks like I'll need to move this component in order to finish my next task, the implementation won't change significantly so your work will still be relevant but it might be worth waiting until the move is done before making any large changes. I'm aiming to get it done this evening.

[Jeffail]

Merged via: https://github.com/benthosdev/benthos/commit/3f064f439bc108a64a4d39d7dd241cc5f5310b33, thanks @monban! In the end I added an explicit field to trigger self signed certs and also put the config fields within a tls namespace.