Alerting: Use URLs in image annotations

[santihernandezc]

Description

This PR prioritizes URLs over tokens in ImageTokenAnnotation, which will allow us to add support for images in the Grafana Cloud Alertmanager.

There are two possibilities for ImageTokenAnnotation:

• The first option is to use the image URL.
• If there's none, the token will be prefixed by token:// and used in the annotation.

Changes

• The ImageTokenAnnotation is a URI that can be either the image URL or the image token with a prefix.
• URLs can be used to look for images, so integrations can still query the DB using the value extracted from ImageTokenAnnotation.
• Instead of adding a *models.Image in state changes, the state manager just adds a URI (string).

Advantages over the previous PR

This PR is meant to replace #66327, and it has a series of advantages over the latter:

• No modifications to our database schemas.
• The Token field is still relevant and used to identify images.
• No duplicate fields (in case of no image URL, we prefixed the Path value and added it to the url column).

Related issue: https://github.com/grafana/alerting-squad/issues/458

[yuri-tceretian]

LGTM