Add configurable Google Chat and custom webhooks

[ameyap94]

Description:
Google Chat incoming webhooks do not accept GoAlert’s generic webhook payload directly. Google Chat expects a Chat message payload, so sending GoAlert’s JSON as-is results in a payload shape mismatch.

This PR adds:

• native Google Chat notification support that formats messages into the {"text":"..."} payload Google Chat expects
• a separate Custom Webhook destination with templated request bodies
• config gates for both destinations so they can be enabled independently of the existing generic webhook contact method

The existing generic webhook contact method continues to use Webhook.Enable.

Custom Webhook examples:

Simple summary text:

{"text":"{{.Summary}}"}

Service alert count:

{"text":"{{.ServiceName}} has {{.Count}} unacknowledged alerts"}

On-call change:

{"text":"{{.ScheduleName}} on-call: {{range .Users}}{{.Name}} {{end}}"}

The template context also exposes fields such as: MessageID, AppName, Type, AlertID, Details, ServiceID, ServiceName, Count, LogEntry, Code, ScheduleID, ScheduleName, ScheduleURL, NewAlertState, Users, and Meta.

Which issue(s) this PR fixes:
None

Screenshots:

Admin configuration

Google Chat Escalation step setup

Escalation steps showing Google Chat destination chip

Notification rule with Google Chat

Notification rule showing Google Chat destination chip

Custom Webhook notification setup

Message previews
Escalation

Notification rule

Describe any introduced user-facing changes:

• Google Chat and Custom Webhook destinations now appear only when their respective config flags are enabled
• Google Chat notifications are sent using a Chat-compatible JSON payload
• Custom Webhook supports templated bodies and optional content type

Describe any introduced API changes:

• Added public config keys:
  • GoogleChat.Enable
  • CustomWebhook.Enable

Additional Info:
Verified with:

• go test ./config ./graphql2 ./notification/webhook ./notification/googlechat ./app -count=1
• node_modules/.bin/tsc -p web/src/app/tsconfig.json --noEmit