Added reauth command to refresh existing agent credentials

[starkmsu]

Added reauth command to refresh existing agent settings with new credentials.
Usage: reauth.cmd or reauth.sh - similar to other CLI commands there. Instead of using 2 commands - 'config remove' and 'config' - to achieve the same result.
Main usecases:

1. After DevFabric redeployment if you have previously working agent configuration and local org has the same URL. Then running reauth cmd wll allow you to just update agent configuration with new PAT preserving the rest of configuration, registering build agent based on previously existing agent configuration.
2. If you need to update agent credential. Existing agent configuration will be updated with new credentials