Unify LambdaContainer and KtFunction

[CedNaru]

While reworking callables, I noticed that the way we two classes we use for registering scripts methods to Godot and for custom callables are extremely similar.
They both use ParameterReader behind the hood to get the arguments from the sharer buffer using a list of VariantConverters and wrap a Kotlin KFunction.
We can probably merge the 2 together in a new abstract class/interface that would be part of the internal library.