Rename the Identity Client interface

[joekr]

What would you like to be added: We should rename the identity.Client interface to something like IdentityClient

Why is this needed: This will keep our client interfaces consistent like the ComputeClient

[joekr]

Starting to work on this and it looks like we should change all the other clients to be more like identity.Client that seems like the more idomatic go way (Name starts with the package name).

https://go.dev/blog/package-names

Avoid repetition. Since client code uses the package name as a prefix when referring to the package contents, the names for those contents need not repeat the package name. The HTTP server provided by the http package is called Server, not HTTPServer. Client code refers to this type as http.Server, so there is no ambiguity.