Added HANA instance restart API

[PakDLiu]

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I understand that this is meant to be an async API and should be using the x-ms-long-running-operation and the x-ms-long-running-operation-options tags. However, our team is trying to demo this restart functionality next month (ran by us, not the customer) and we didn't have enough time to implement the async operations API. The intent of the demo is to execute this API (Either via the portal or CLI), and we will continuously call GET till it returns the correct state. This spec will be updated with the correct long-running tags and with operation APIs before we go public. My lead is aware of all of this and has approved it. Thank you for your understanding.

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The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#3251

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[azuresdkci]

Can one of the admins verify this patch?

[lagalbra]

Other than examples, which I think should definitely be added, LGTM

[sarangan12]

WHat will happen if the restart fails?

[PakDLiu]

It will return a 404 if the instance is not found, 400 if the instance is not in a valid power state (ie: already restarting) and 500 if something on the server went wrong.

I noticed in the compute VM spec only has 200 and 202, do you not have to set all the return codes? or only the valid ones? Same on the GET VM, only 200.