Add `wp cache flush-group` for flushing a cache group

[janw-me]

Needs extra input for testing.

I don't know how to create/fake a cache implementation for better behat testing.

Fixes https://github.com/wp-cli/cache-command/issues/80

[janw-me]

@danielbachhuber the failed tests are valid, the errors should trigger with older WP versions. How do we handle this? is a @require-wp-6.1 the way to go?

[danielbachhuber]

@janw-me Yes, please:

1. Create entirely separate scenarios.
2. Add @require-wp-6.1 to the scenarios with the new functionality.
3. Add @less-than-wp-6.1 to a scenario that tests the error case.

[spacedmonkey]

@danielbachhuber Where I do personally agree, I think delete group would be better. But there are plugins that used that already so we could not use it. Let's use flush-group, as this is the naming in core, even through it sucks, it sucks in the WordPress way.