Add support for configuring Restate cluster nodes

[pcholakov]

The Operator should assist with Restate Cluster configuration and bootstrap. Currently (as of v2.1.0), the extent of the support it offers to create a headless service for the node-to-node port 5122, which makes pod addresses resolvable over local DNS by other pods.

Actually using this still requires a lot of explicit Restate config, which the operator does not attempt to help with. At a minimum, one has to export something like:

RESTATE_ADVERTISED_ADDRESS="http://$(POD_NAME).restate-cluster.$(POD_NAMESPACE).svc.cluster.local:5122"

And use code to enumerate and construct the peer addresses on startup:

for i in $(seq 0 $((REPLICAS - 1))); do
  if [ $i -gt 0 ]; then
    addresses="${addresses},"
  fi
  addresses="${addresses}\"http://restate/-${i}.restate-cluster.${POD_NAMESPACE}.svc.cluster.local:5122\""
done

export RESTATE_METADATA_CLIENT__ADDRESSES="[$addresses]"

Additionally, the operator could assist with provisioning the cluster once the prerequisite number of nodes are up and available when using Raft metadata stores. (Automatic provisioning is a bad idea there as startup delays or partitions can cause multiple clusters to be provisioned independently.)

[tillrohrmann]

The last bit has been solved via #55. If the cloud is already using the new operator, then maybe it's a good idea to enable cluster provisioning by the operator.

[pcholakov]

Nice, great to know! This will be super handy to enable self-service cluster upgrades in Cloud.