ApiManagement: Add support for Backend Entity (#705)

* ApiManagement: Add support for Backend Entity

* Review fixes

* Defined $top and $skip to Global section and consolidated definition. Line ending in description

* Fixed the default value from "true" to true

* Fix issue #684 with file name from apimmanagement to apimanagement.json

#684

* Add IdentityProvider REST Api to the Spec for documentation

* Rename BackendContract and Add Quota Counter API

* Renames to types

Author: solankisamir