Add support for 'when' filter attributes

[hartmantis]

Description

Because 'when' is also a reserved keyword in Ruby, that can't be the name of the new attribute on the filter resource. However, 'days' is the only valid attribute within the 'when' scope, so that seemed a decent option.

Motivation and Context

There is currently no way to place data in the 'when' attribute scope of a filter other than using the sensu_json_file resource directly.

How Has This Been Tested?

ChefSpec tests have been added and confirmed to pass.

Screenshots (if appropriate):

N/A

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.

[cwjohnston]

Hi @RoboticCheese, thanks for this excellent pull request! I appreciate your efforts to add a new feature, complete with tests and a documentation example! 😻

At first glance I think this looks good, and I believe we may be able to include it in the impending 3.2.0 release.

[portertech]