Pass tokens in gRPC metadata to duo-workflow-service"

This MR does the following

1. Adds new flags with default values from environment variables
2. Prepares grpc metadata based on the flags or env variables and sends them to duo workflow service.

How to verify

1. Run duo workflow service. Go through the MR description to prepare token.
2. Provide the flags to the executor either as command line arguments or from environment variables
3. Verify that authentication succeeds.

Closes #2 (closed)