Alerting: Schedule a shim implementation for recording rules

[alexweav]

What is this feature?

Right now, if a recording rule is present in the database it'll be scheduled as an Alert rule. This will immediately fail because recording rules lack some of the alert rule fields.

Here we introduce an alternate rule implementation for recording rules. It's left as a shim for now that doesn't do anything, but it doesn't fail either.

This unblocks work on metrics and tracking health for recording rules without needing to write their full implementation yet. Makes future work easier to parallelize.

Which issue(s) does this PR fix?:

n/a

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[rwwiv]

LGTM