cmd/viewer: generate method wrappers

The `tailscale.com/cmd/viewer` utility needs to be updated to generate methods that wrap the underlying type's methods with value receivers. The generated methods should transform the return parameters of the underlying methods to make them read-only. Additionally, the utility should generate `{Type}ViewOf` functions to be used as [`MakeViewFn`](https://github.com/tailscale/tailscale/blob/20562a4fb9c2af417c44b3ab63a826d9d0f6b6b2/cmd/viewer/viewer.go#L80)s by the `cmd/viewer` itself.

From https://github.com/tailscale/tailscale/pull/12830#discussion_r1722595393