Multi Cluster Support

As discussed in #1307 it would be nice to have cluster a context setting for each controller.

Why?
Currently a operator can only a act on a single context. Traditionally that's fine as an operator often orchestrates only resources in the very cluster its deployed it self. In this scenario the operator gets its identity natively from the kubernetes context and is statically configured for its own cluster.
In contrast to that we have a use case where we manage multiple clusters with on operator which is deployed to a meta/central cluster. The operator still manages resources within this meta cluster but to instantiate the desired application it will also manage child/runtime clusters.

Example:

The control cluster manages a CRD which has been instantiated twice. The operator picks them up and deploys resources to fulfil the desired resource. To do that the operator has to deploy the application into multiple clusters which are e.g. physically separated and therefore can't have a single identity or are separated due to availability needs or to test new versions of kuberentes etc. Therefore a full replication of the resources is created in each cluster and the operator watches all of them.

Control Cluster
┌──────────────────────────────┐  Cluster A
│                              │ ┌──────────────────────────────────────────┐
│    Namespace-crds            │ │  Namespace-A            Namespace-B      │
│    ┌──────────────┐      ┌───┼─┤►┌─────────────┐       ┌─────────────┐    │
│    │              │      │   │ │ │             │       │             │    │
│    │ CRD-A        │      │   │ │ │ Deployment  │       │  Deployment │    │
│    │ CRD-B        │      │   │ │ │             │       │             │    │
│    │              │      │   │ │ └─────────────┘       └─────────────┘    │
│    └─────▲────────┘      │   │ │                                          │
│          │               │   │ └──────────────────────────────────────────┘
│          │ Reconcile     │   │
│          │               │   │  Cluster B
│     ┌────┴────────┬──────┘   │  ┌──────────────────────────────────────────┐
│     │             │          │  │  Namespace-A            Namespace-B      │
│     │  Operator   ├──────────┼──┤►┌─────────────┐       ┌─────────────┐    │
│     │             │  Create/ │  │ │             │       │             │    │
│     └─────────────┘  Reconcile  │ │ Deployment  │       │  Deployment │    │
│                              │  │ │             │       │             │    │
│                              │  │ └─────────────┘       └─────────────┘    │
│                              │  │                                          │
└──────────────────────────────┘  └──────────────────────────────────────────┘

Goals:

• add a context setting for the controller annotation so context can be in addition to the namespace for each reconciler individually
• add startup probe to ensure all desired context are available

首先查看 issue #1307 和链接的 PR #2499，以了解之前的处理方式。未指定文件或测试；完成的标准是支持按 controller 设置上下文注解，并添加一个启动探针来验证所有所需的上下文都可用。