Correctly load client or server specific decorators from classpath (#1592)

The current approach that attempts to downcast never worked; all Rust
decorators were being loaded, and the cast was doing nothing, because
the generic type parameter is erased at runtime.

Attempting to downcast a generic class `C<T>` to `C<U>` where `U: T` is
not possible to do in Kotlin (and presumably all JVM-based languages)
_at runtime_. Not even when using reified type parameters of inline
functions. See https://kotlinlang.org/docs/generics.html#type-erasure
for details.

This commit thus goes for another approach, suggested in the linked
Stack Overflow question [0]: add a method to the loaded classes that
signals at runtime the generic type parameter (`ClientCodegenContext` or
`ServerCodegenContext`) they can work with, in order to filter them.

This commit also simplifies the way the Python server project loads the Python
server-specific decorators, by deleting the combined decorator
`PythonServerCodegenDecorator`, which was being loaded from the classpath, and
instead directly using `CombinedCodegenDecorator` and passing it the Python
server-specific decorators in the `extras` parameter.

[0]: https://stackoverflow.com/questions/5451734/loading-generic-service-implementations-via-java-util-serviceloader