fix(dashboard): add support for custom datasource

[kevincali]

Motivation

I was unable to use a metrics datasource that is not prometheus.

What

Add support for custom datasources in the dashboard.

Why

I was unable to use a metrics datasource that is not prometheus.

How

Added curly braces around the variable.

Verification Steps

1. Deploy dashboard

apiVersion: grafana.integreatly.org/v1beta1
kind: GrafanaDashboard
metadata:
  name: keycloak
spec:
  folder: Keycloak
  instanceSelector:
    matchLabels:
      dashboards: grafana
  datasources:
    - inputName: Datasource
      datasourceName: victoriametrics
  url: https://raw.githubusercontent.com/kevincali/keycloak-metrics-spi/refs/heads/master/grafana/10441.json

2. Be happy that the dashboard shows data :D

Checklist:

• Code has been tested locally by PR requester
• Changes have been successfully verified by another team member

Progress

• Finished task

Additional Notes

This was only tested with victoriametrics as a datasource.
I assume this won't break anything with prometheus.