Offchain Workers: Example Pallet (#4989)

* Example of offchain worker pallet.

* Fix compilation issues.

* Use serde_json to parse JSON.

* Add some basic tests.

* Working on docs.

* Fix compilation

* Finish docs for signed.

* Work on unsigned send.

* Add some tests and missing docs.

* Add example of StorageValueRef

* Add weight.

* Extra \n

* Fix im-online test.

* Bump runtime.

* Fix tests.

* Apply suggestions from code review

Co-Authored-By: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>
Co-Authored-By: Gavin Wood <gavin@parity.io>

* Address review comments.

Co-authored-by: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>
Co-authored-by: Gavin Wood <github@gavwood.com>