`alerts/for` should have a configuration option to disable default values check

[alerts/for](https://cloudflare.github.io/pint/checks/alerts/for.html) states:

>This check will warn if an alert rule uses invalid for or keep_firing_for value or if it passes default value that can be removed to simplify rule.

I'd like to disable the check that warns about passing a default value, or omitting `for`, so you can get`for: 0m` by default, but want to keep the useful and critical invalid `for:` settings.

So a configuration option to disable the default value check would be helpful.

Start with the linked alerts/for check documentation and trace how pint configures individual checks. Add an option that disables warnings for omitted or default for values while preserving warnings for invalid settings. Update the relevant documentation and tests, and verify both warning paths.

Written by the indexing model from the issue text.