[resource-manager] more structural support in `Extension.ExternalResource`

[ArcturusZhang]

I think this is because we are not able to define an external resource for this:
/providers/Microsoft.Management/managementGroups/{managementGroupId}/providers/Microsoft.Billing/billingPeriods/{billingPeriodName}

Possible syntax could be:

alias ManagementGroupBillingPeriod = Extension.ExternalResource<
	"Microsoft.Billing",
	"billingPeriods",
	"billingPeriodName",
	ParentType = ManagementGroup
;

where ParentType here should be able to reference another external resource so that this could be recusive.

Originally posted by @ArcturusZhang in #37162

The uri of the operation looks like this:

/providers/Microsoft.Management/managementGroups/{managementGroupId}/providers/Microsoft.Billing/billingPeriods/{billingPeriodName}/providers/Microsoft.Consumption/aggregatedCost

We should consider this as an extension provider action of /providers/Microsoft.Consumption/aggregatedCost on external resource /providers/Microsoft.Management/managementGroups/{managementGroupId}/providers/Microsoft.Billing/billingPeriods/{billingPeriodName}

If we plan just to fix for this case, it is also fine, in this case, we should add ManagementGroup into ParentType's valid values, and this could look like:

alias ManagementGroupBillingPeriod = Extension.ExternalResource<
  "Microsoft.Billing",
  "billingPeriods",
  "billingPeriodName",
  ParentType = "ManagementGroup"
>;

[markcowl]

The biggest issue here is that this is a non-resource operation. The operation returns a single "resource", but has no name parameter in the path to go with the resource type.

Generally, I would expect this to be sorted with provider operations. We could provide a providerOperations template for this pattern if it occurs in lots of other RPs, but for now, using a low-level operation using static routing is the best solution.

If this is desired to be sorted with something like 'AggregatedCost', then we would need a decorator to allow doing this to a standard operation, or we could use the 'list' operation template using a custom body, but this just seems like the wrong template to use.

[markcowl]

It is impractical to create a model template that could be recursive over any number of providers using @autoRoute, this is because each provider uses the same path parameter name and, using a template, we must specify the path parameter names as part of the template. If this were a function rather than a template, or if we used mutators to calculate the parameters, then this might be possible, but IMO this is an anti-pattern, and using static routes is far simpler in that case.

[markcowl]

We could imagine having a template that would allow this (by specifying the provider property in a mutator), but before we would make a change like that, we would need to know how many specs follow this pattern. Otherwise, if there are only a few, a low-level operation with a resourceOperation decorator seems like the bets bet.

[pshao25]

Synced offline, it's more like a partial resource design issue, but low priority. Circle back later.