Handle missing imports with a context manager and a wraper for classes and objects

[davorrunje]

As the dependencies become larger, many optional dependencies have mutual conflicts. To generate API documentation, we should be able to load all submodules with only the required dependencies. One way to accomplish this is as follows:

def optional_import_block(modules: Union[str, Iterable[str]], dep_target: str) -> Result:
  ...

with check_for optional_import_block() as result:
   import some_optional_module

skip_tests = result.is_successful

@require_optional_import("some_optional_module", "some-optional-dep-target")
class MyClass():
    def __init__():
        o = some_optional_module.SomeClass()

If some_optional_module module is installed, the construction of MyClass will work as expected. If not, it will raise autogen.exceptions.ImportError("MIssing imported module 'some_module', please install it using 'pip install ag2["some-optional-dep-target"]'").

[davorrunje]

This is needed for #528