Add support for parsing filters from string or serde_json::Value

[onelson]

The motivation for having a type-safe(ish) filter API is to help promote correctness at compile-time when authoring filters, but there are plenty of use cases for parsing filters at runtime too.

To get there, we should provide support for parsing filters from strings, as well as from a serde_json::Value. The filter types have a custom Serialize impl which means we currently only know how to generate json, not parse it. See if we can rework the serde aspect of this to help make both Serialize and Deserialize work.