azurerm_api_management_subscription: make user_id optional

[patst]

We want to create subscriptions per application. Therefore we don't want to assign a specific user to a subscription.

The details are described in #8923 .

• The owner_id is optional (see https://docs.microsoft.com/en-gb/azure/api-management/api-management-subscriptions)
• added a new test for the userless subscription.
• I moved the schemaz.SchemaApiManagementChildID() struct to the resource file because it is the only place it is referenced.

make testacc TEST='./azurerm/internal/services/apimanagement/api_management_subscription_resource_test.go' TESTARGS='-run=TestAccApiManagementSubscription_complete' TESTTIMEOUT='120m'
==> Checking that code complies with gofmt requirements...
==> Checking that Custom Timeouts are used...
==> Checking that acceptance test packages are used...
TF_ACC=1 go test ./azurerm/internal/services/apimanagement/api_management_subscription_resource_test.go -v -run=TestAccApiManagementSubscription_complete -timeout 120m -ldflags="-X=github.com/terraform-providers/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccApiManagementSubscription_complete
=== PAUSE TestAccApiManagementSubscription_complete
=== CONT  TestAccApiManagementSubscription_complete
--- PASS: TestAccApiManagementSubscription_complete (2601.34s)
PASS
ok      command-line-arguments  2602.981s

make testacc TEST='./azurerm/internal/services/apimanagement/api_management_subscription_resource_test.go' TESTARGS='-run=TestAccApiManagementSubscription_withoutUser' TESTTIMEOUT='120m'
==> Checking that code complies with gofmt requirements...
==> Checking that Custom Timeouts are used...
==> Checking that acceptance test packages are used...
TF_ACC=1 go test ./azurerm/internal/services/apimanagement/api_management_subscription_resource_test.go -v -run=TestAccApiManagementSubscription_withoutUser -timeout 120m -ldflags="-X=github.com/terraform-providers/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccApiManagementSubscription_withoutUser
=== PAUSE TestAccApiManagementSubscription_withoutUser
=== CONT  TestAccApiManagementSubscription_withoutUser
--- PASS: TestAccApiManagementSubscription_withoutUser (2657.30s)
PASS
ok      command-line-arguments  2659.031s

Fixes #8923

[tombuildsstuff]

LGTM 👍

[jackofallops]

Tests looking fine, Failures are transient / unrelated:

[ghost]

This has been released in version 2.52.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.52.0"
}
# ... other configuration ...

[ghost]

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