Alertmanager: Load additional templates for Grafana receivers

[stevesg]

Grafana uses a slightly different set of default templates, so we need to load those for some of receivers to work.

ts=2024-06-20T09:01:38.840935785Z caller=log.go:44 level=warn component=MultiTenantAlertmanager user=702 logger=ngalert.notifier.webhook notifierUID=db8e8538-3b70-4444-a830-14ad4d2e4cd6 msg="failed to template webhook message" error="template: :1:12: executing \"\" at <{{template \"default.title\" .}}>: template \"default.title\" not defined"

I think it would be safest initially to only load these if we've decided to use the Grafana configuration. If we're not using it, then don't. In the future when configurations are merged, we can decide how to handle this.

@santihernandezc has already implemented this in his POC branch - code for reference:

if err := tmpl.Parse(strings.NewReader(alertingTemplates.DefaultTemplateString)); err != nil {
	return err
}

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1. Alertmanager: Add default Grafana template string #8465
   component/alertmanager +1