Add powershell APIs for Partition merge and Throughput redistribution

[amar-sagare]

Description

Checklist

• [x ] I have read the Submitting Changes section of CONTRIBUTING.md
• [x ] The title of the PR is clear and informative
• [x ] The appropriate ChangeLog.md file(s) has been updated:
  • 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 -- no new version header should be added
• [x ] The PR does not introduce breaking changes
• [x ] If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • [x ] the markdown help files have been regenerated using the commands listed here

[wyunchi-ms]

/azp run azure-powershell - security-tools

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[amar-sagare]

/azp run azure-powershell - security-tools

[azure-pipelines]

Commenter does not have sufficient privileges for PR 18247 in repo Azure/azure-powershell

[wyunchi-ms]

/azp run azure-powershell - windows-powershell

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[wyunchi-ms]

/azp run azure-powershell - security-tools

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).