Introduce a `Append` method in `ResourceIdentifier` class and use it in singleton resource construction

[ArcturusZhang]

When constructing singleton resources, we are using this:

autorest.csharp/test/TestProjects/SingletonResource/Generated/ParentResource.cs

Line 95 in 1bbca80

return new SingletonResource(Client, new ResourceIdentifier(Id.ToString() + "/singletonResources/current"));

We could introduce a Append method in the ResourceIdentifier class to reduce the validation and parsing cost in resource ids.
We already have them:

• https://github.com/Azure/azure-sdk-for-net/blob/01183276806f43a2df4a2a0f00302813ad85bb80/sdk/core/Azure.Core/src/ResourceIdentifier.cs#L553
• https://github.com/Azure/azure-sdk-for-net/blob/01183276806f43a2df4a2a0f00302813ad85bb80/sdk/core/Azure.Core/src/ResourceIdentifier.cs#L568