[design]: Remove need to use legacy and private decorators in Network spec

[markcowl]

To prevent a name clash with a custom ProxyResource, network specs cannot use standard resource templates for new resources. We should create a mechanism where private dcorators and suppressions are not needed for new ntwork resources. This could be:

• Fixes to SDK emitters to allow the standard and custom ProxyResource in network
• Allowing suppression in the base type to apply to new resources
• Adding new templates that only require suppression in the base type and provide any private decorators
  example: https://github.com/Azure/azure-rest-api-specs/pull/42717/changes#diff-ec5e52cdc9d434e2bde0ebd57ae1230d36275f3f638e3577065fcd9fda30d16e