Add --upgrade_warnings_as_errors flag

[Jasha10]

This PR is to close #1689.

The main contributions are:

• create a new subclass HydraUpgradeWarning of UserWarning
• In deprecation warnings, use category=HydraUpgradeWarning
• Add a command line flag --upgrade_warnings_as_errors that causes warnings.warn("...", category=HydraUpgradeWarning) to raise an error instead of a warning.

[omry]

I was thinking about something a bit different:

1. create a common function to call to issue a warning.
2. That function can check the presence of an environment variable to decide if to trigger an error as well (sys.exit, maybe just an exception).

This way, we will also be able to replace that function at runtime to add additional logic to the warning (such as custom logging in fbcode).

What do you think?

[Jasha10]

What do you think?

Sounds reasonable to me. I've opened #1703 with an initial implementation.