Replace Statefulsets with customized controller

## Problem
Currently etcd-operator uses StatefulSets to manage the etcd member/PODs. In happy cases, it's working well so far. But when it comes to repair a failed member/POD, it's hard to do it with StatefulSet.

Statefulsets assigns an ordinal Index for each POD, and always scales down from the highest ordinal. If member 1 is corrupt but member 0 and 2 are healthy, Statefulsets forces you to remove member-2 first.

Also currently the controller manages `Statefulset.spec.replicas`, the StatefulSet only manages "how many pods to run". It knows nothing about the etcd membership. This introduces a reconciliation gap: we must constantly reconcile STS-replica-count vs etcd-member-count.

## Proposal

The solution is to introduce a customzed controller to manage the etcd members/PODs. We discussed multiple solutions multiple times in our community meetings.
- we manage POD directly, each etcd member maps 1-to-1 to a Pod owned directly by EtcdCluster.
- we manage deployment or statefulset directly, and each etcd member maps to 1-to-1 to a Statefulset or deployment owned by EtcdCluster. For each statefulset or deployment, the replica is 1.

We believe the first solution is the most flexible. Ideally, we should deliver a PoC for each solution and decide to use which one later. But we might not have that bandwidth. Let's start from solution 1, and deliver a PoC for it first. If it's confirmed that it doesn't need huge effort and easy to understand & maintain. We might proceed with it.

Note we haven't released v1.0 yet, so we're happy to discard the existing design and implementation and start over in future if necessary.

## Requirements

We don't need to deliver a complete & perfect solution for now. Instead, let's do it step by step. The first step is just to replace the StatefulSet with a customized controller as mentioned in solution 1 above. We don't add new feature in the first step, we just keep the existing functionalities:
- **Users are able to create a brand new etcd cluster of any size**
- **Users are able scale in & out an existing etcd cluster**
- Users are able to configure the certificate providers: auto, cert-manager or disabled
- Users are able to configure a storageClass to integrate with an existing CSI driver
- Users are able to customze the etcd options

cc @hakman @ivanvc @jberkus @nwnt