Allow for multiple JSON providers

In some cases there may be multiple providers, for instance one that comes with the Maven dependencies, and one that I explicitly want to use. To allow the loading of a specific provider, `JsonProvider` should have a `getName()` method that returns the name of the provider. The name can be use to select a specific provider.

Start with the JsonProvider API and the provider-loading behavior described in the issue. Define how getName() identifies a provider and how callers select that name, then verify that multiple providers can coexist and the requested provider is loaded.

Written by the indexing model from the issue text.