add FetchContext FetchOpt

[fredwangwang]

Proposed change

Currently the new js.Fetch can only take a max wait time through FetchMaxWait. I have a use case where I want to cancel my fetch (and return whatever has been fetched) as soon as the context has been cancelled. This would allows better control and intergation with calling function that has a context that passed in.

Use case

Better integration with other components that works with Context.

Contribution

sure thing.