Add key_vault_reference_identity_id attribute for azurerm_app_service_slot

[heoelri]

This PR is related to PR #13720 and #13962 and adds support for key_vault_reference_identity_id to azurerm_app_service_slot. This work is based on @patst's work in PR #13720.

=== RUN   TestAccAppServiceSlot_keyVaultUserAssignedIdentity
=== PAUSE TestAccAppServiceSlot_keyVaultUserAssignedIdentity
=== CONT  TestAccAppServiceSlot_keyVaultUserAssignedIdentity
--- PASS: TestAccAppServiceSlot_keyVaultUserAssignedIdentity (233.09s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/web   233.130s

This PR closes #13968

[katbyte]

Thanks @heoelri - LGTM 🙌

[github-actions]

This functionality has been released in v2.84.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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