scope() function to support multi-scope modules

[flanakin]

Is your feature request related to a problem? Please describe.
Some resources support multiple scopes, like Cost Management budgets and scheduled actions. The modules for these features are identical, except for any specific handling around the scope. In some cases, we need specific logic based on the scope ID, but this requires different logic for each scope:

// For subscriptions...
var scopeId = subscription().id
// For resource groups...
var scopeId = resourceGroup().id
// For management groups...
var scopeId = managementGroup().id

Since these functions all work differently for each scope, it's not possible to reuse the same code for each scoped module.

Describe the solution you'd like
Create a scope() function that is an alias for the scope the deployment is run for.

[alex-frankel]

Related to #788

[anthony-c-martin]

Also related to #4698