[FEATURE] Support dapr-proxy mode

[webup]

Please describe your usage scenario

Support one-dapr-sidecar-per-function mode, see OpenFunction/OpenFunction#360.

Check OpenFunction/functions-framework#21 for new env vars added.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

This task is in todo list of OpenFunction 0.8.0 release, see also OpenFunction/OpenFunction#363.

🐱 type: maintenance, priority: high