Add support for workflow invocations (#42)

* Add `workflows` module that contains the "invoke" and "invoke for
external user" cases already present in the existing TypeScript SDK
* Document the new functionality in the reference doc
* Bump major version (release candidate)

* Remove the response parsing logic since it's free-form
* Take over the Proxy clients so that they expose a nicer interface

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Co-authored-by: fern-api <115122769+fern-api[bot]@users.noreply.github.com>
Co-authored-by: Jay Vercellone <jay@pipedream.com>