Add gateways API

[yoramsinger]

Latest improvements:

MSFT employees can try out our new experience at OpenAPI Hub - one location for using our validation tools and finding your workflow.

Contribution checklist:

• I have reviewed the documentation for the workflow.
• Validation tools were run on swagger spec(s) and have all been fixed in this PR.
• The OpenAPI Hub was used for checking validation status and next steps.

ARM API Review Checklist

• Service team MUST add the "WaitForARMFeedback" label if the management plane API changes fall into one of the below categories.
• adding/removing APIs.
• adding/removing properties.
• adding/removing API-version.
• adding a new service in Azure.

Failure to comply may result in delays for manifest application. Note this does not apply to data plane APIs.

• If you are blocked on ARM review and want to get the PR merged urgently, please get the ARM oncall for reviews (RP Manifest Approvers team under Azure Resource Manager service) from IcM and reach out to them.
  Please follow the link to find more details on API review process.

[AutorestCI]

Automation for azure-sdk-for-go

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-go#4065

[AutorestCI]

Automation for azure-sdk-for-python

Nothing to generate for azure-sdk-for-python

[AutorestCI]

Automation for azure-sdk-for-js

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-js#686

[AutorestCI]

Automation for azure-sdk-for-ruby

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-ruby#2218

[AutorestCI]

Automation for azure-sdk-for-java

Nothing to generate for azure-sdk-for-java

[AutorestCI]

Automation for azure-sdk-for-node

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-node#4299

[azuresdkci]

Can one of the admins verify this patch?

[jianghaolu]

@azuresdkci add to whitelist