Add support for running code snippets using the "playground" feature.

[sumanthvrao]

This is definitely not a near-term priority feature right now, but just as a heads-up to a feature worth integrating into all clients in the future.

zulip/zulip#16432 (which is the tracking issue) details support for "playground" links which allows one to easily open the contents of a code block in an external playground. (E.g: Rust code being opened in the rust online playground).

With PR zulip/zulip#16496 the backend settings support for configuring realm playgrounds has been implemented. Once merged, the API documentation for this will be found at https://zulip.com/api/add-playground and https://zulip.com/api/remove-playground.