Add new API object BackendRuntime for expandability

[kerthcet]

What would you like to be added:

Right now, once we add a new backend support, we'll have to implement the Backend interface, and provide the default values, which means each time we want to support a new backend, we'll have to modify the controller logic. This is not that expandability considering Kubernetes has CRD intended for that. What's more, the default values can not be modified.

So let's make them a CRD Backend and export these default values by default, once admin wants to change the values, they can modify them directly.

This required the support of helm installation.

Why is this needed:

Better expandability.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[kerthcet]

/kind feature

[kerthcet]

/milestone v0.1.0

[kerthcet]

This is not that hurry because we can always add an backendRuntime when we needed. However, we should consider this after we have metrics support to see how to design the API.

[kerthcet]

/milestone clear

[kerthcet]

/milestone v0.1.0