[ReleasePR search] [Azure Cognitive Search][Resource Manager]: Add Swagger API for 2021-04-01-Preview API version (#1751)

Create to sync Azure/azure-rest-api-specs#14313
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/master...AzureSDKAutomation:sdkAuto/search?expand=1)

Author: msftbot[bot]