Add client parameter -certfile to provide certificate to trust

[mporsch]

When reading up on the TLS option InsecureSkipVerify (set via command line argument -insecure) I found lots of hints that this is more insecure than users may expect and should not be used in production code.
An alternative approach is to provide a certificate (chain) to trust. This is e.g. explained in https://forfuncsake.github.io/post/2017/08/trust-extra-ca-cert-in-go-app/

Generally asking; is there interest to integrate this?
Alternatively the user can place the certificate in the OS' global certificate store (e.g. /etc/ssl/certificates/). But it might be undesirable to put system-wide trust onto some self-signed certificate the user provided for one application.

I have not yet looked into test run results, but the initial prototype test works as expected against a self-signed server certificate:

# wstunnel cli -tunnel wss://sensor:sensor@192.168.100.233:8080 -token 02:A2:5A:01:00:1A -server http://127.0.0.1
INFO[02-10|14:34:58] WStunnel starting                        pkg=WStuncli
INFO[02-10|14:34:58] Setting WS keep-alive                    timeout=30s
INFO[02-10|14:34:58] wstunnel dev - 2022-02-10 12:00:00 - devel pkg=WStuncli
INFO[02-10|14:34:58] Dispatching to external server(s)        pkg=WStuncli server=http://127.0.0.1 regexp=nil
INFO[02-10|14:34:58] WS   Opening                             pkg=WStuncli url=wss://192.168.100.233:8080/_tunnel token=02:A2...
EROR[02-10|14:34:58] Error opening connection                 pkg=WStuncli err="x509: certificate signed by unknown authority" info=

# wstunnel cli -tunnel wss://sensor:sensor@192.168.100.233:8080 -token 02:A2:5A:01:00:1A -server http://127.0.0.1 --insecure
INFO[02-10|14:35:03] WStunnel starting                        pkg=WStuncli
INFO[02-10|14:35:03] Setting WS keep-alive                    timeout=30s
INFO[02-10|14:35:03] wstunnel dev - 2022-02-10 12:00:00 - devel pkg=WStuncli
INFO[02-10|14:35:03] Accepting unverified SSL certs from local HTTPS servers pkg=WStuncli
INFO[02-10|14:35:03] Dispatching to external server(s)        pkg=WStuncli server=http://127.0.0.1 regexp=nil
INFO[02-10|14:35:03] WS   Opening                             pkg=WStuncli url=wss://192.168.100.233:8080/_tunnel token=02:A2...
INFO[02-10|14:35:03] WS   ready                               pkg=WStuncli ws=0x197a000 server=http://127.0.0.1
INFO[02-10|14:35:03] pinger starting                          pkg=WStuncli ws=0x197a000

# wstunnel cli -tunnel wss://sensor:sensor@192.168.100.233:8080 -token 02:A2:5A:01:00:1A -server http://127.0.0.1 -certfile /tmp/cert.pem 
INFO[02-10|14:35:07] WStunnel starting                        pkg=WStuncli
INFO[02-10|14:35:07] Setting WS keep-alive                    timeout=30s
INFO[02-10|14:35:07] wstunnel dev - 2022-02-10 12:00:00 - devel pkg=WStuncli
INFO[02-10|14:35:07] Dispatching to external server(s)        pkg=WStuncli server=http://127.0.0.1 regexp=nil
INFO[02-10|14:35:07] WS   Opening                             pkg=WStuncli url=wss://192.168.100.233:8080/_tunnel token=02:A2...
INFO[02-10|14:35:07] WS   ready                               pkg=WStuncli ws=0x198c000 server=http://127.0.0.1
INFO[02-10|14:35:07] pinger starting                          pkg=WStuncli ws=0x198c000