feat: Allow to override metrics' host project

[juli4n]

This change allows to specify the host project of a metric in the metric spec, so that the adapter can fetch metrics from a different project than the one where the adapter is running.

This is useful in scenarios where metrics are stored in a central project, but the HPA is running in a different project.

The host project can be specified by adding the resource.labels.metrics_host_project_id label to the metric selector in the HPA definition.

For external metrics, the legacy resource.labels.project_id label is also supported for backward compatibility.

How to use:

To fetch a metric from a different project, add the resource.labels.metrics_host_project_id label to the metric selector in the HPA definition.

Example for a custom metric:

apiVersion: autoscaling/v2beta2
kind: HorizontalPodAutoscaler
metadata:
  name: custom-metric-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: my-app
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Pods
    pods:
      metric:
        name: my-metric
        selector:
          matchLabels:
            "resource.labels.metrics_host_project_id": "my-metrics-project"
      target:
        type: AverageValue
        averageValue: 10

Fixes #979