Add support for adding custom Contexts to ContextCommand

[mahaloz]

Is your feature request related to a problem? Please describe.
Often when making new commands for a debugger, you want to add a custom Context pane, like on other
debuggers such as pwndbg. It helps the reverser make a more tuned workflow for analyzing a binary. A way to add custom context panes from users would make this possible!

Describe the solution you'd like
I'd like a register_external_context_pane, just like the register_external_command function that allows you to add a new pane to be printed in the ContextCommand. The parameters can be pane_title, pane_display_function.

Describe alternatives you've considered
Alt: hooking on each break event. It's not a great solution because it is not a part of the formal Context you see.

Additional context
Yes, I've already implemented most of it in my PR which I will link in the next comment. I would love input on it :). The biggest concern I have is how I update settings for the context. Check it out.