Queries should be immutable value objects

Problem/Motivation

Project Browser source plugins have a getProjects() method which accepts one optional argument - an array of query parameters. The query parameters are documented on the interface, but there's no enforcement of them and they're not reliable.

Proposed resolution

Instead of an array, let's make the queries into an immutable value object which can be created from the request. This will give us a strong interface for the queries themselves, as well as type safety and the ability to do strong validation logic.