What is this feature?

This PR adds a new feature toggle alertingRuleNotificationMessageSectionExtension that allows extending the notification message section in alerting rule forms.

It also creates a simple extension API in notificationMessageSectionExtensions/extensions.ts
It modifies AnnotationsStep to render the extensible notification message section bellow the current content.

Why do we need this feature?

[Add a description of the problem the feature is trying to solve.]

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes #

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.