[C4GT] provide 'update' method for stream contents

[amarts]

What is this enhancement about?

Today, we have stream.create() and stream.revoke() methods added in cord.js SDK. We need to build proper interfaces and methods for stream.update() extrinsic, so we can get started with full fledged integration with current studio and any other tools like sunbird RC.

How to test?

Enhance yarn demo to perform stream.update, and make sure we see stream.updated event on chain using explorer UI.

What are the things to consider?

• content module will have to get a proper 'update()' method too, which should be passed for stream module.

[NiranjanAP]

Created another issue with proper formatting needed for displaying on C4GT portal