* Have webhooks refresh their certs automatically
Fixes partially #5272
In 2.9 we introduced the ability for providing the certs for `proxy-injector` and `sp-validator` through some external means like cert-manager, through the new helm setting `externalSecret`.
We forgot however to have those services watch changes in their secrets, so whenever they were rotated they would fail with a cert error, with the only workaround being to restart those pods to pick the new secrets.
This addresses that by first abstracting out `FsCredsWatcher` from the identity controller, which now lives under `pkg/tls`.
The webhook's logic in `launcher.go` no longer reads the certs before starting the https server, moving that instead into `server.go` which in a similar way as identity will receive events from `FsCredsWatcher` and update `Server.cert`. We're leveraging `http.Server.TLSConfig.GetCertificate` which allows us to provide a function that will return the current cert for every incoming request.
### How to test
```bash
# Create some root cert
$ step certificate create linkerd-proxy-injector.linkerd.svc ca.crt ca.key \
--profile root-ca --no-password --insecure --san linkerd-proxy-injector.linkerd.svc
# configure injector's caBundle to be that root cert
$ cat > linkerd-overrides.yaml << EOF
proxyInjector:
externalSecret: true
caBundle: |
< ca.crt contents>
EOF
# Install linkerd. The injector won't start untill we create the secret below
$ bin/linkerd install --controller-log-level debug --config linkerd-overrides.yaml | k apply -f -
# Generate an intermediatery cert with short lifespan
step certificate create linkerd-proxy-injector.linkerd.svc ca-int.crt ca-int.key --ca ca.crt --ca-key ca.key --profile intermediate-ca --not-after 4m --no-password --insecure --san linkerd-proxy-injector.linkerd.svc
# Create the secret using that intermediate cert
$ kubectl create secret tls \
linkerd-proxy-injector-k8s-tls \
--cert=ca-int.crt \
--key=ca-int.key \
--namespace=linkerd
# start following the injector log
$ k -n linkerd logs -f -l linkerd.io/control-plane-component=proxy-injector -c proxy-injector
# Inject emojivoto. The pods should be injected normally
$ bin/linkerd inject https://run.linkerd.io/emojivoto.yml | kubectl apply -f -
# Wait about 5 minutes and delete a pod
$ k -n emojivoto delete po -l app=emoji-svc
# You'll see it won't be injected, and something like "remote error: tls: bad certificate" will appear in the injector logs.
# Regenerate the intermediate cert
$ step certificate create linkerd-proxy-injector.linkerd.svc ca-int.crt ca-int.key --ca ca.crt --ca-key ca.key --profile intermediate-ca --not-after 4m --no-password --insecure --san linkerd-proxy-injector.linkerd.svc
# Delete the secret and recreate it
$ k -n linkerd delete secret linkerd-proxy-injector-k8s-tls
$ kubectl create secret tls \
linkerd-proxy-injector-k8s-tls \
--cert=ca-int.crt \
--key=ca-int.key \
--namespace=linkerd
# Wait a couple of minutes and you'll see some filesystem events in the injector log along with a "Certificate has been updated" entry
# Then delete the pod again and you'll see it gets injected this time
$ k -n emojivoto delete po -l app=emoji-svc
```
Alejandro Pedraza