Add new helper method /meta/aksCheckCredentials
Add new helper method /meta/aksCheckCredentials

Add a new POST endpoint /meta/aksCheckCredentials that accepts AKS
credentials, minus tenant ID, as a POST request, and checks whether they
are valid. This allows the UI to validate the user's input when a cloud
credential or cluster is being created before the cloud credential
exists.

(cherry picked from commit 34abf4ac5dafdba018ff07e32e2e78d147cce6a5)

rancher/rancher,rancher/rancher,rancher/rancher,rancher/rancher