Add  Server API method: UserInfo

Add a method to the UserManagerAPI which takes a username and returns a struct of user information (username and display name at this stage).
so this *looks* to me very much like a `UserInfo`. What's the thinking behind having separate types? The params package really ought to be expressing the language we're using in the api, not just a grab-bag of ad-hoc structs.

NOTE: there are certainly reasons to choose to split the types. I'm not sure the ones I can think of are quite good enough to justify it, but I want to be sure we've considered them.