[Issue Alert Creation Toast] Optionally Allow Cancellation of Saving an Alert Rule

### Problem Statement

Sometimes it can take far to long to look up a Slack channel. In these cases the individual making the alert must exit the context, come back and make a new alert from scratch.

The workaround is to use a Slack Channel ID but this also requires the customer to start from scratch.

### Solution Brainstorm

When the toast has taken more than a few seconds to lookup the channel consider surfacing:

1) A cancel request button.
2) instructions to explicitly use the channel ID when the request is slow.

### Product Area

Alerts

No files, tests, or entry points are named. Start by tracing alert rule creation and the toast flow where Slack channel lookup occurs, then determine whether cancellation or channel-ID guidance is feasible. Done criteria are not defined because the issue presents alternatives rather than a selected solution.

Written by the indexing model from the issue text.