Support autoscaling

[hxietkg]

Add a new block nodepool.spec.auto_scaling, which contains enabled, min_count and max_count to enable/disable the autoscaling on a nodepool.

1. What this PR does / why we need it: TKGS supports auto-scaling from 80u3 (tkg-service 3.0.0), this MR is to expose the feature in terraform cli. To support this feature from TMC side, a new member in nodepool's spec was added, it contains: enabled: true or false to enable/disable auto-scaling on a nodepool min_count: minimum number of nodes during auto-scaling max_count: maximum number of nodes during auto-scaling

test done:

• 1. create a nodepool with {enabled: true, min_count:1, max_count:2}, to verify the nodepool being created successfully,
• 2. change a nodepool from {enabled: true, min_count:1, max_count:2} to {enabled: false, min_count:0, max_count:0}, to verify auto-scaling being disabled successfully.
• 3. change a nodepool from {enabled: false, min_count:0, max_count:0} to {enabled: true, min_count:1, max_count:2}, to verify auto-scaling being enabled successfully.
• 4. change a nodepool from {enabled: true, min_count:1, max_count:2} to {enabled: true, min_count:2, max_count:3}, to verify the change applied to cluster nodepool.

5. Which issue(s) this PR fixes

6. Additional information

7. Special notes for your reviewer