New extension API

[EggAllocationService]

With the removal of the public extension API in e99fe00 (now requires subclassing LibmapperDevice), it might be a good idea to replace it with something more idiomatic.

Potential ideas:

• Inspector panel where users can drag-and-drop extension classes?
• Automatically search loaded classes for ITypeConverters and IPropertyExtractors?

[EggAllocationService]

Update: The inspector panel option isn't super viable due to how ScriptableObject works. It'd be a big headache for developers to have to add all their extensions as an asset and a script. Looking into automatic discovery now.

[EggAllocationService]

After testing it out I don't think auto discovery is the right move either, it freezes the whole game for a few seconds on startup and I don't think we want our plugin to be responsible for that.