App Service / App Service Slot / Function Apps - support for HTTPS Only

[tombuildsstuff]

Adds support for https_only to App Service, App Service Slot, Function App resources and the App Service Data Source

Fixes #756

[tombuildsstuff]

@JunyiYi apologies, I've only just seen that you've been assigned to this issue after I'd opened this PR 🤦‍♂️

[tombuildsstuff]

App Service tests pass:

$ acctests azurerm TestAccAzureRMAppService_httpsOnly
=== RUN   TestAccAzureRMAppService_httpsOnly
--- PASS: TestAccAzureRMAppService_httpsOnly (113.75s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	113.795s

App Service Slot tests pass:

$ acctests azurerm TestAccAzureRMAppServiceSlot_httpsOnly
=== RUN   TestAccAzureRMAppServiceSlot_httpsOnly
--- PASS: TestAccAzureRMAppServiceSlot_httpsOnly (111.72s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	111.758s

Function App tests pass:

$ acctests azurerm TestAccAzureRMFunctionApp_httpsOnly
=== RUN   TestAccAzureRMFunctionApp_httpsOnly
--- PASS: TestAccAzureRMFunctionApp_httpsOnly (104.63s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	104.683s

[mbfrahry]

LGTM

[ghost]

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