[resource-manager]: The `etag` properties in `@azure-tools/typespec-azure-resource-manager` should have the type of `ETag` from azure core

Now the `etag` properties are modeled as `string` - which is not entirely correct, we have the `ETag` scalar defined in `@azure-tools/typespec-azure-core` therefore we should use them.
For instance: https://github.com/Azure/typespec-azure/blob/13dd00287e232498d90234613396500d5a983db2/packages/typespec-azure-resource-manager/lib/models.tsp#L264

Read packages/typespec-azure-resource-manager/lib/models.tsp at the referenced etag properties, then inspect the ETag scalar in @azure-tools/typespec-azure-core. Update the resource-manager etag properties to use ETag rather than string, and verify the affected TypeSpec models and generated output remain correct.

Written by the indexing model from the issue text.