Out-of-the-box support for URL/URI fields in clients

[jmartisk]

with automatic translation between URL/URI and String

[t1]

See #1164

[t1]

I think a specific solution would not be sufficient; we should have a more generic approach.

There used to be some code that also would consider java.net.URL and java.net.URI to be valid scalar wrappers, given that they have a String-only constructor. But we then removed it due to #270. We could re-add it with the additional requirement that the class be either on a list of allowed wrapper classes (a built-in list containing, e.g., these two, plus a custom config option) or annotated accordingly (maybe reusing the server-side @AdaptToScalar).