[ARM]: Support resource with multiple paths

[markcowl]

• Create a canonical pattern for an ARM resource that can be hosted under multiple paths
  • Resources at each of these paths have the same resource schema and may have the same or different operations
  • example: PolicyDefinition (subscription, resource group)
  • Resources may also have recursive parentage (ManagementGroups)

[pshao25]

We have several (maybe we can say many) RPs in which a same resource model defined is used in different clients. For example, this client and this client share the same resource model Site.

/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Web/sites/{name}
/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Web/sites/{name}/slots/{slot}

The only way we could write its TypeSpec now seems like this.

[pshao25]

#1282 should be handled simultaneously.