This endpoint will allow clients to create a playground entry
containing the name, pygments language and url_prefix for the
playground of their choice.
Introduced the `do_*` function in-charge of creating the entry in
the model. Handling the process of sending events which will be
done in a follow up commit.
Added the openAPI format data to zulip.yaml for POST
/realm/playgrounds. Also added python and curl examples for using
the endpoint in its markdown documented (add-playground.md).
Tests added.
Sumanth V Rao