feat(config): Use a proxy dialer

[thib92]

This PR creates a Go net/proxy Dialer with the FromEnvironment method and passes it in the Kafka client configuration under Net.Proxy. This allows users to configure a proxy client from environment variables (for example using ALL_PROXY). If no environment variable is set, the proxy will be transparent, making this PR backwards-compatible.

For more safety, we could place this code under a condition on an extra configuration key (use_proxy, or specify explicitly the proxy configuration in the provider configuration). However, I've inspired this from the PostgreSQL provider's proxy support

I've tested this manually on one of our projects that uses this provider. Specifying a ALL_PROXY=localhost:1080 environment variable makes the provider pas through the local proxy. Unsetting the variable makes it behave as before.

Fixes #147

[Mongey]

Thanks!