Use a logging mechanism that allows greater output control.

Requirements that this mechanism meets:
 * users should be able to change the verbosity programmatically (ag.set_verbosity)
 * users should be able to change the verbosity without touching the code (AUTOGRAPH_VERBOSITY env variable)
 * different levels of verbosity should be supported, ala vlog
 * the mechanism should work in interactive prompts like Jupyter by mirroring the log messages to console
All these controls are global (at least for now).

PiperOrigin-RevId: 230514195