Improve extensibility of type generators

[brantburnett]

Motivation

Make it easier for extensions to control type generation. In particular, allow an extension to override the default type generation without requiring modification after code generation using an enricher.

Modifications

• Allow more than one ITypeGeneratorFactory to be registered per category. These are then tried in order, based on Priority and order of registration, and the first that doesn't return null is chosen. Built-in factories are registered with a priority of 0 but added last.
• Add new service registration methods that support registering multiple ITypeGeneratorFactory instances, obsolete the old methods.
• Drop the TGeneratorCategory generic type parameter from all related interfaces and classes. Instead, using the support for keyed service registration added in .NET 8.
• Update all instances of type generator categories to use the new approach.
• Make NoopTypeGenerator and NoopTypeGeneratorFactory internal.

Results

• Extensions can now register a type generator factory which takes precendence over the built-in type generator factory, but they can still fallback to built-in behaviors by returning null.
• Extensions can much more easily create custom generator categories using string keys instead of complicated generics.

Breaking Change

Existing extensions which register custom generator categories must be rewritten to use string based keys instead of generic types.

For example:

- .TryAddTypeGeneratorFactory<OpenApiSchema, MyCategory, MyFactory>();
+ .AddTypeGeneratorFactory<OpenApiSchema, MyFactory>("MyCategory");

- internal class MyFactory : ITypeGeneratorFactory<OpenApiSchema, MyCategory>
+ internal class MyFactory : ITypeGeneratorFactory<OpenApiSchema>

- public MyConsumer(ITypeGeneratorRegistry<OpenApiSchema, MyCategory> registry)
+ public MyConsumer([FromKeyedServices("MyCategory")] ITypeGeneratorRegistry<OpenApiSchema> registry)

[brantburnett]

/merge