Move alert source-specific attributes onto payload class

What does this MR do and why?

Moves a few methods around alert integrations logic. This is a refactor only, and it does not modify any behavior.

This MR paves the way for !95827 (closed), so I can keep that MR smaller.

How to set up and validate locally

Send generic alerts

1. In a project with maintainer+ permissions, nav to Settings > Monitor > Alerts to create/turn on an HTTP alert integration
   • Skip custom mapping fields for fastest integration creation
2. Select 'Send test alert` to send an alert to your integration

   { "title": "This is a new test alert", "fingerprint": "fingerprint" }

3. Visit Monitor > Alerts to see the new alert, and select Activity to see the system notes which are generated from new methods

Send recovery alerts

1. Go back to Settings > Monitor > Alerts to send another test alert, and include the appropriate resolving attributes (end_time for HTTP integrations) { "title": "This is a new test alert", "fingerprint": "fingerprint", "end_time": "2022-08-30T11:22:40Z" }
2. Visit Monitor > Alerts to view your original alert -> it should be resolved & have new system notes under "Activity" tab

Send Prometheus alerts

1. Alerts from prometheus can be sent to either a "Prometheus" integration, or a generic HTTP integration. They'll be processed the same either way.
2. Firing prometheus payload:

   {
     "version" : "4",
     "groupKey": null,
     "status": "firing",
     "receiver": "",
     "groupLabels": {},
     "commonLabels": {},
     "commonAnnotations": {},
     "externalURL": "", 
     "alerts": [{
       "startsAt": "2022-08-30T11:22:40Z", 
       "generatorURL": "http://host?g0.expr=up", 
       "endsAt": null,
       "status": "firing",
       "labels": {
         "gitlab_environment_name": "production"
       }, 
       "annotations": {}
     }]
   }

3. Prometheus recovery payload:

   {
     "version" : "4",
     "groupKey": null,
     "status": "resolved",
     "receiver": "",
     "groupLabels": {},
     "commonLabels": {},
     "commonAnnotations": {},
     "externalURL": "", 
     "alerts": [{
       "startsAt": "2022-08-30T11:22:40Z", 
       "generatorURL": "http://host?g0.expr=up", 
       "endsAt": "2022-08-30T18:22:40Z",
       "status": "resolved",
       "labels": {
         "gitlab_environment_name": "production"
       }, 
       "annotations": {}
     }]
   }

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.