Improvement/Add proxy config to config.yaml

[blotus]

Currently, the only way to configure crowdsec/cscli to use a proxy is to set HTTP_PROXY or HTTPS_PROXY in the env.

While it mostly works, there are some usecases where it does not (for example, if the user runs sudo cscli ...., sudo won't keep those env vars).

We should add a new option in config.yaml to allow the user to set a proxy without messing with env vars.

This would also ease the setup on windows (as adding env vars to a service requires a registry edit).

[LaurenceJJones]

Assign @mmetc for investigation stage