Support multi-cluster with helm plugin

[absoludity]

Description of the change

This PR updates the GRPC registration function for plugins so that it receives the clusters configuration. This enables plugins to make decisions based on the clusters configuration, which is required for the Helm plugin as it behaves slightly differently for the cluster on which Kubeapps is installed.
Even though the Helm plugin only requires the kubeapps cluster name, I've passed the complete config for generality... perhaps we should just pass the kubeapps cluster name for now (though that makes assumptions about other plugins later needs).

It then updates the Helm plugin so that:

• Requests for packages available in a namespace of other clusters only ever returns the global available packages (ie. those from the kubeapps namespace on the kubeapps cluster), and
• Requests for installed packages on a cluster (or namespace of a cluster) use the config for that cluster.

Example, after installing apache on the second cluster:

grpcurl -H "$auth_header" -plaintext -d '{"context": {"cluster": "second-cluster", "namespace": "default"}}' localhost:8080 kubeappsapis.plugins.helm.packages.v1alpha1.HelmPackagesService.GetInstalledPackageSummaries
{
  "installedPackageSummaries": [
    {
      "installedPackageRef": {
        "context": {
          "cluster": "second-cluster",
          "namespace": "default"
        },
        "identifier": "apache-second-cluster"
      },
      "name": "apache-second-cluster",
      "pkgVersionReference": {
        "version": "8.6.0"
      },
      "currentPkgVersion": "8.6.0",
      "currentAppVersion": "2.4.48",
      "iconUrl": "https://bitnami.com/assets/stacks/apache/img/apache-stack-220x234.png",
      "pkgDisplayName": "apache",
      "shortDescription": "Chart for Apache HTTP Server",
      "latestPkgVersion": "8.6.0",
      "status": {
        "ready": true,
        "reason": "STATUS_REASON_INSTALLED",
        "userReason": "deployed"
      }
    }
  ]
}

Benefits

We can continue to support the existing multicluster functionality when using the new API.

Possible drawbacks

Applicable issues

• fixes [kubeapps-apis] Implement support for cluster in packaging API #3278

Additional information

[gfichtenholt]

Looked at everything except server_test, which looked had way too many changes to analyze properly I trust you on this one. If tests are passing +1 from me.

[absoludity]

Looked at everything except server_test, which looked had way too many changes to analyze properly I trust you on this one. If tests are passing +1 from me.

Yep, tests are passing and the real-life test works as per the description. Thanks.