Implement token auth for cp/dp connections

[kate-osborn]

Parent issue: #292

The data plane should authenticate with the control plane upon connection using a long-lived Kubernetes token. The control plane should verify the token using Kubernetes TokenReview API. For more information, see https://github.com/nginxinc/nginx-kubernetes-gateway/blob/main/design/control-data-plane-separation/design.md#authorization.

A/C:

• The data plane agent attaches its long-lived Kubernetes Service Account Token to its connection request to the control plane
• The control plane verifies that the agent's token is valid and corresponds to the expected Kubernetes Service account
• If the token is not valid, the control plane does not register the agent and returns an error
• If the token is valid, the control plane allows the agent to register

Aha! Link: https://nginx.aha.io/features/NKG-38