[Hello-Java.Base] Add support for Java Callable Wrappers (dotnet#1315)

Context: 2a1e180

Update `samples/Hello-Java.Base` to use `Java.Interop.Sdk` (2a1e180),
 so that it (kinda, sorta) "automagically" gets support for generating
Java Callable Wrappers as part of its build.

Add a custom `Java.Lang.Object` subclass and use it.

Update `JreTypeManager` and `JreRuntimeOptions` so that type mappings
can be more easily provided without requiring a `JreTypeManager`
subclass, as Java.Interop.Sdk" is not fleshed out enough to provide
that information itself.  (This also simplifies `TestJVM`.)