Support passing proxy configuration using environment variables or command flags

[jordeu]

Currently, the Agent can be used with a proxy, but it requires non-standard (non-intuitive) command-line flags.

Example:

tw-agent -Dmicronaut.http.client.proxy-address=10.0.0.1:666 -Dmicronaut.http.client.proxy-type=http ...

Accepting widely used environment variables like http_proxy and https_proxy will make proxy support more user-friendly. To clarify this, some explicit tw-agent flags can be added to the command line.