PR Summary

Add basic logging for error cases in the Azure agent.

• Use Serilog as it supports file logging. I considered Microsoft.Extension.Logging which has built-in logging provider for Event Source, but decided to not use it because it's easier to share a file log for diagnosis purposes.
• The logging is configured to be rolling over by day, and it uses async writing to the log file.
• The logging enabled so far is error only, but we can add more information/debug loggings for better view of the code operation.
• We will need to allow a user to disable logging in the agent's configuration file. This is not done in this PR.