If `IServiceCollection.AddContentTypeBasedSerializationSupport` is called prior to `IServiceCollection.AddContent`, the generic factory for content handler dispatchers will not be registered in the service collection

There are two means by which the instance of `ContentFactory` can be added to the `IServiceCollection`:
- `ContentHandlingJsonServiceCollectionExtensions.AddContentTypeBasedSerializationSupport`
- `ContentFactoryServiceCollectionExtensions.AddContent`

The first adds our implementation `IJsonSerializationSettingsProvider`, an instance of `ContentFactory` and our custom `JsonConverter` implementations, `ContentTypeConverter` and `ContentEnvelopeConverter`.

The second is used to add content types to the `ContentFactory`. It also registers the generic factory for content handler dispatchers (`IContentHandlerDispatcher<>` implemented by `ContentHandlerDispatcher<>`).

If during service container setup, `AddContentTypeBasedSerializationSupport` is called prior to `AddContent`, the generic factory for content handler dispatchers will not be registered due to the conditional logic in that method.

We should either:
1. Add extra conditional logic to `AddContent` to ensure the factory is registered even if the `ContentFactory` has already been added to the collection, or
2. Move the factory registration into its own method so that it can be explicitly added to the container if needed.

(My preference is for the second option, for two reasons:
- usage of our content type-based serialization is much more common than usage of our content handler dispatcher approach, so it makes sense to make it opt-in rather than automatically registering it with the serialization support.
- it is not at all obvious that you need to call `AddContent` in order to register the factory)

Note that whilst this is clearly a breaking change, I have not yet found anywhere we're actually using the `IContentHandlerDispatcher<>` so this isn't a huge issue.

Start at IServiceCollection.AddContentTypeBasedSerializationSupport and AddContent, then trace the conditional registration of IContentHandlerDispatcher<> implemented by ContentHandlerDispatcher<>. Resolve the registration approach and verify both service-registration call orders; done means the generic factory is available when the supported setup is used.

Written by the indexing model from the issue text.