Allow multiple prefix queries in API

In #1835 we added support for the `prefix` query parameter to the normal GET APIs. In some cases it would be nice to allow multiple `prefix` parameters and have the API return all of them in one merged object.

For example, this was done in #1836 to allow users to specify multiple settings to `apiclient get`. Settings are merged, with ones coming from later prefixes replacing those from earlier prefixes.

I imagine that for most programmatic uses it would be simpler to fetch the entire `Model` and access the specific keys needed, but for human interfaces it's useful. (We should probably make sure there are more use cases than `apiclient get` before adding any complexity.)