Support scaling with Spot instances for cost saving

[kerthcet]

What would you like to be added:

See https://github.com/spotinst as an example, which means we should support multi cloud providers.

Why is this needed:

Cost saving for users.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[kerthcet]

/kind question

[kerthcet]

Not only for cost saving, but also to proactive autoscaling just like https://github.com/kaito-project/kaito

/milestone v0.2.0

[kerthcet]

/kind feature
/remove-kind question

[kerthcet]

cc @carlory

[carlory]

/assign

[kerthcet]

Any progress on this? We're approaching the deadline, end of this monty clearly.