Send demo alert with dynamic payload and get demo payload example on private api

[iskhakov]

What this PR does

• Added demo_alert_payload field to /api/internal/v1/alert_receive_channels/ GET endpoint

curl 'http://localhost:3000/api/plugin-proxy/grafana-oncall-app/api/internal/v1/alert_receive_channels/'

• Added demo_alert_payload field to /api/internal/v1/alert_receive_channels/***/send_demo_alert/ POST endpoint

curl 'http://localhost:3000/api/plugin-proxy/grafana-oncall-app/api/internal/v1/alert_receive_channels/***/send_demo_alert/' \
  -XPOST -H 'Content-Type: application/json' \
  --data-raw '{"demo_alert_payload":{"alerts":[{"a":"b"}]}}' \
  --compressed

Which issue(s) this PR fixes

Checklist

• Unit, integration, and e2e (if applicable) tests updated
• Documentation added (or pr:no public docs PR label added if not required)
• CHANGELOG.md updated (or pr:no changelog PR label added if not required)

[iskhakov]

todo return with integrations/