Extend the TypeFunction API to be more flexible

[cvolzke]

Custom TypeFunctions are presently registered by java class. If I want to create my own TypeFunction, I must implement getAcceptedTypes and return exactly the classes I support creating graphql types for:

Collection<Class<?>> getAcceptedTypes();

This isn't a very flexible design and doesn't allow for type functions for:

• Map<ApiEnum, *>. I want to generate a type where the key is ApiEnum, but I don't want to build types for other Map objects
• All implementors of a specific interface
• A type that has so and so annotation(s)
• etc

To work around this, we've created our own 'type function' concept that has canBuildType and buildType methods. Instead of looking up the type function from a Map<Class, TypeFunction>, we iterate through all known TypeFunction objects calling canBuildType until there is a match. This satisfies the use cases listed above.