Add support for cron shortcuts

[mattchenderson]

Dapr does a nice thing where they support a few alternatives to cron expressions, such as "@daily" or "@every 5m". See https://docs.dapr.io/reference/components-reference/supported-bindings/cron/

This is a great way to simplify setup for common cases. Supporting something like this would make it easier for folks to get started with TimerTrigger. Getting started templates / examples should probably move to a default here. If someone needs something more advanced, they can switch to use the classic cron expression pattern.

[mattchenderson]

Noting a requirement specific to Functions - all SKUs should get this. Either the scaling infrastructure will need to update accordingly or something needs to be done in the metadata generation / synctriggers to convert these shortcuts into the right cron expressions

[mathewc]

So we'd be doing a limited set of conversions, e.g.:

"@every 15s" => "*/15 * * * * *"
"@every 20m" => "0 */20 * * * *"
"@every 2h" => "0 0 */2 * * *"
"@daily" => "0 0 0 * * *"
"@hourly" => "0 0 * * * *"

I think we'd just have a shared converter that both the Timer extension and ScaleController use to do these conversions, leaving the actual schedule expression unmodified in the trigger payload/metadata.