cmd/k8s-operator: operator can create subnetrouter (#9505)

* k8s-operator,cmd/k8s-operator,Makefile,scripts,.github/workflows: add Connector kube CRD.

Connector CRD allows users to configure the Tailscale Kubernetes operator
to deploy a subnet router to expose cluster CIDRs or
other CIDRs available from within the cluster
to their tailnet.

Also adds various CRD related machinery to
generate CRD YAML, deep copy implementations etc.

Engineers will now have to run
'make kube-generate-all` after changing kube files
to ensure that all generated files are up to date.

* cmd/k8s-operator,k8s-operator: reconcile Connector resources

Reconcile Connector resources, create/delete subnetrouter resources in response to changes to Connector(s).

Connector reconciler will not be started unless
ENABLE_CONNECTOR env var is set to true.
This means that users who don't want to use the alpha
Connector custom resource don't have to install the Connector
CRD to their cluster.
For users who do want to use it the flow is:
- install the CRD
- install the operator (via Helm chart or using static manifests).
For Helm users set .values.enableConnector to true, for static
manifest users, set ENABLE_CONNECTOR to true in the static manifest.

Updates tailscale/tailscale#502


Signed-off-by: Irbe Krumina <irbe@tailscale.com>

go.mod

@@ -103,6 +103,7 @@ require (
 	k8s.io/client-go v0.28.2
 	nhooyr.io/websocket v1.8.7
 	sigs.k8s.io/controller-runtime v0.16.2
+	sigs.k8s.io/controller-tools v0.13.0
 	sigs.k8s.io/yaml v1.3.0
 	software.sslmate.com/src/go-pkcs12 v0.2.1
 )
@@ -111,6 +112,7 @@ require (
 	github.com/Microsoft/go-winio v0.6.1 // indirect
 	github.com/dave/astrid v0.0.0-20170323122508-8c2895878b14 // indirect
 	github.com/dave/brenda v1.1.0 // indirect
+	github.com/gobuffalo/flect v1.0.2 // indirect
 	github.com/google/gnostic-models v0.6.9-0.20230804172637-c7be7c783f49 // indirect
 	github.com/gorilla/securecookie v1.1.1 // indirect
 )
@@ -325,7 +327,7 @@ require (
 	github.com/ssgreg/nlreturn/v2 v2.2.1 // indirect
 	github.com/stbenjam/no-sprintf-host-port v0.1.1 // indirect
 	github.com/stretchr/objx v0.5.0 // indirect
-	github.com/stretchr/testify v1.8.4 // indirect
+	github.com/stretchr/testify v1.8.4
 	github.com/subosito/gotenv v1.4.2 // indirect
 	github.com/t-yuki/gocover-cobertura v0.0.0-20180217150009-aaee18c8195c // indirect
 	github.com/tailscale/go-winio v0.0.0-20231025203758-c4f33415bf55