Extensions API Review (#3409) (#3544)

* Extensions API Draft Review (#3409)

Extension API Draft Spec 

Co-authored-by: Diana Qu <[email protected]>
Co-authored-by: David Risney <[email protected]>

* typo

Co-authored-by: Raymond Chen <[email protected]>

* No need to capture member variable

Co-authored-by: Raymond Chen <[email protected]>

* Add noexcept

Co-authored-by: Raymond Chen <[email protected]>

* sample code change

Co-authored-by: Raymond Chen <[email protected]>

* add await

Co-authored-by: Raymond Chen <[email protected]>

* Doc Typo change

Co-authored-by: Raymond Chen <[email protected]>

* typo

Co-authored-by: FrankC-msft <[email protected]>

* Fix API review comment

* Update more comment

---------

Co-authored-by: Diana Qu <[email protected]>
Co-authored-by: David Risney <[email protected]>
Co-authored-by: Raymond Chen <[email protected]>
Co-authored-by: FrankC-msft <[email protected]>