This PR introduces a gRPC service to retrieve the pods available using the `search_as_roles` configured in the user's roles.
This gRPC service receives a TLS identity and manipulates it to access the Teleport Kubernetes Proxy and retrieve the pods available for a hypothetical user that has the `search_as_roles` defined for its roles.
The response does not leak the certificate nor the pod details other than `name`, `namespace` and `labels`.
```mermaid
sequenceDiagram
participant User as User
participant gRPC as grpc Server
participant Auth as Auth Server
participant KubeProxy as Kube Proxy
participant KubeSVC as Kube Service
User->>+gRPC: ListKubeResources(cluster,namespace)
Note left of gRPC: Includes search_as_roles <br/> and preview_as_roles <br/>into identity!
gRPC-->>+Auth: Sign cert with ProcessKubeCSR
Auth-->>-gRPC: Signed certificate
gRPC->>+KubeProxy: list pods "v1.../namespaces/{namespace}/pods"
KubeProxy-->>+KubeSVC: list pods "v1.../namespaces/{namespace}/pods"
KubeSVC-->>-KubeProxy: filtered response
Note left of KubeSVC: Applies Pod RBAC filtering!
KubeProxy-->>-gRPC: response
gRPC->>-User:Return name, namespace and labels
```
Part of #21107 and #19573
