Microsoft.LoadTestService loadTests: It's not possible to use 'existing' resource references for encryption properties

**Bicep version**
Bicep CLI version 0.23.1

**Describe the bug**
My attempt to add encryption to a load test template failed as its encryption property does not allow me to use 'existing' resource references.

As soon as any of the properties are not a direct string reference, but for example an output of an 'existing' key vault resource reference, the template starts failing with an error such as:


**To Reproduce**
Try to deploy using this [template](https://github.com/sebassem/bicep-registry-modules/blob/avm-load-test-service/avm/res/load-test-service/load-test/tests/e2e/user-assigned-cmk-encryption/main.test.bicep)

**Additional context**
Similar to Azure/bicep#7066

Start with the linked user-assigned-cmk-encryption main.test.bicep template and compare the failure with the behavior described in Azure/bicep#7066. Trace the Microsoft.LoadTestService loadTests encryption type handling; done means a template can use values produced by an existing Key Vault resource and deploy successfully.

Written by the indexing model from the issue text.