Add a curl-like command that uses the zed context

[jzelinskie]

The idea is to add a command that's a low-level way to construct requests, but reusing the zed context.

An example of this functionality is this plugin for heroku:

heroku api PATCH /apps/myapp/config-vars --body '{"FOO": "bar"}'

Thanks to @marccampbell for the idea: https://twitter.com/mccode/status/1663615667034275840