Update ControlMessage to hold arbitrary Python objects & update MessageMeta to copy & slice

[yczhang-nv]

Description

• For ControlMessage, use a specialization of nlohmann::basic_json to hold arbitrary Python objects for m_metadata & m_tasks

• Implement dataframe slicing method for MessageMeta which is equivalent to copy_meta_ranges() in MultiMessage. The method will do copy & slicing to the original dataframe, instead of sharing the ownership.

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[mdemoret-nv]

• set_metadata and get_metadata should return a JSONValues object.
• Same with Tasks

[mdemoret-nv]

Looking good but lets build on this. Other things we should be thinking of here:

• How could we possibly break this code? Lets make tests to try and break it and fix the implementation if they succeed. Try different types of objects, overwriting objects, deleting objects, etc.
• Can we verify that the python object was destroyed if we erase the key that is holding it?
• What other utility functions would developers want when working with these objects?

[mdemoret-nv]

Update the type casters in include/morpheus/pybind11/json.hpp to use your new type.

[mdemoret-nv]

/merge