Add support for service-mirror selectors

[adleong]

Fixes #4481

We add support for the source cluster to now specify which services from the target cluster should be exported by using a
label selector rather than looking for the mirror.linkerd.io/gateway and mirror.linkerd.io/gateway-ns annotations. Further context for this change is described in linkerd/rfc#31

The Link resource spec now has a selector field which contains a label selector. All services in the target cluster which match this selector will be mirrored. This selector can be set by using the --selector flag to the multicluster link command and follows the same format as the --selector flag does in kubectl. The selector can alternatively be set by editing the Link resource directly. If the selector flag to multicluster link is left unspecified, it defaults to the selector: mirror.linkerd.io/exported.

Some example usage:

1. Using the default selector:

linkerd --context=target multicluster link --cluster-name=foo | kubectl --context=source apply -f -
kubectl --context=target -n emojivoto label svc/web-svc mirror.linkerd.io/exported=true
# the web-svc service is now mirrored to the source cluster

2. Using a custom selector

linkerd --context=target multicluster link --cluster-name=foo -l exportme=please | kubectl --context=source apply -f -
kubectl --context=target -n emojivoto label svc/web-svc exportme=please
# the web-svc service is now mirrored to the source cluster

[zaharidichev]

LGTM! It is nice that this results in completely removing the export-service command

[zaharidichev]

Maybe worth putting a comment that this piece of code is to exercise the serde rountrip

[kleimkuhler]

Looks good! Tested this locally with two k3d clusters and everything works as expected.

Edit: Also A+ branch name 👌