[AKS] support Mode in 'New-AzAksNodePool' and 'Update-AzAksNodePool'

[YanaXu]

Description

Support Mode in 'New-AzAksNodePool' and 'Update-AzAksNodePool'.
After this solution:

• New-AzAksCluster
  • Support NodePoolMode.
  • Since AzPS now only supports creating one nodepool when creating a aks cluster, and AKS requires at least one nodepool is system mode, New-AzAksCluster only supports NodePoolMode=System, which is the default value if it's not set.
• Set-AzAksCluster
  • Support NodePoolMode.
  • It's only valid when there is more than 1 pool. It requires NodeName in this case.
• New-AzAksNodePool
  • Support Mode.
• Update-AzAksNodePool
  • Support Mode.

Checklist

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD select appropriate branch. Cmdlets from Autorest.PowerShell should go to generation branch.
• SHOULD make the title of PR clear and informative, and in the present imperative tense.
• SHOULD update ChangeLog.md file(s) appropriately
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense. Add changelog in description section if PR goes into generation branch.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD have approved design review for the changes in this repository (Microsoft internal only) with following situations
  • Create new module from scratch
  • Create new resource types which are not easy to conform to Azure PowerShell Design Guidelines
  • Create new resource type which name doesn't use module name as prefix
  • Have design question before implementation
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT introduce breaking changes in Az minor release except preview version.
• SHOULD NOT adjust version of module manually in pull request

[wyunchi-ms]

Set-AzAksCluster and New-AzAksCluster is not updated in this PR.