Feature request: Add Identity-related APIs for LogicApp

[LogicApp APIs](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/logic/resource-manager/Microsoft.Logic/preview/2018-07-01-preview/examples/Operations_List.json)

My use-case:
1) Deploy LogicApp using ARM template and Azure Go SDK (this exists)
2) **EnableSystemIdentity()** - Enable or disable "System Assigned" Identity for the LogicApp (*New*)
3) **ListIdentity()** - this will return the OID corresponding to the System Assigned Identity (*New*)
The OID will be used for Role Assignment to enable the LogicApp to GET/POST to Azure REST endpoints

Note: ListIdentity() would be similar to ListCallbackURL() to provide system-generated IDs to the caller.

Please add support for the items in Bold.

Thanks,
Sourabh

Start with the LogicApp API specification and the linked preview example, especially Operations_List.json, then compare the requested methods with ListCallbackURL. Define the specification changes needed for enabling or disabling a system-assigned identity and returning its OID, and verify that both requested operations are represented consistently.

Written by the indexing model from the issue text.