rfc: Rework alerts

[Blaumaus]

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• I have searched Swetrix feature requests and there are no duplicates

Describe the solution you'd like?

I think Alerts could be improved a lot if we reworked them to be more like Workflows / Sequences for automation.

To do this, we should allow:

1. Support for more channels were to send the alerts, configurable per alert, not per user account. Meaning that in user / project settings users should be able to connect their channels (Telegram, Discord, Webhook, etc.), and we able to enable / disable them in the alert settings
2. The messages sent should be customisable. For example, like it's done in email sequences on services like Postmark - where users can select variables. On Swetrix example:

• for custom event alerts users would be able to craft custom messages with variable like "User from {{country}} have done {{count}} {{custom event name}} in the last {{hour}} hours. Session link - {{session link}}" or something like that
• for pageviews alerts it should also be supported

3. A bit overreach, but it would be AWESOME if we supported building proper sequences with some editor for it. Like building a sequence - website has X events in the last 24 hours -> yes (do this, do that); -> no (do this, end sequence)

Additional context

No response

How important is this feature to you?

1 (Not too important)

[Blaumaus]

The new alerts / workflows should also include support for push notifications as a channel - #419