cmd/viewer,types/views,various: avoid allocations in pointer field getters whenever possible

In this PR, we add a generic views.ValuePointer type that can be used as a view for pointers
to basic types and struct types that do not require deep cloning and do not have corresponding
view types. Its Get/GetOk methods return stack-allocated shallow copies of the underlying value.

We then update the cmd/viewer codegen to produce getters that return either concrete views
when available or ValuePointer views when not, for pointer fields in generated view types.
This allows us to avoid unnecessary allocations compared to returning pointers to newly
allocated shallow copies.

Updates #14570

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -282,7 +282,7 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 							MachineKey: p.Machine().String(),
 							NodeKey:    p.Key().String(),
 						},
-						Online:              p.Online(),
+						Online:              p.Online().Clone(),
 						TailscaleSSHEnabled: p.Hostinfo().TailscaleSSHEnabled(),
 					}
 				}),