Add `resourceType` scalar in `typespec-azure-core` library

[ArcturusZhang]

We also need a resourceType scalar to make a specific scalar to be the arm resource type. DotNet SDK will generate resourceType into Azure.Core.ResourceType. Other languages could simply generate it as string.

The definition could be

scalar resourceType extends string;

And the TrackedResource, ProxyResource template needs to be updated accordingly - their type property should be resourceType.

[markcowl]

Also, this should change the type of the allowedResourceTypes array in the armResourceIdentifier template

[markcowl]

est: 5