Support fetch api request stream

[oott123]

This fixes #669 for connect transport. Maybe we should support it for grpc transport too.

[CLAassistant]

All committers have signed the CLA.

[timostamm]

Thanks for the PR, @oott123!

As mentioned in #669 (comment), we'll need quite a bit more plumbing to get this merged though, especially regarding test coverage. We are kindly sponsored by the BrowserStack Open Source program, so it should be possible to get good confidence with tests for the happy path and the unhappy paths.

[oott123]

I have added crosstests which taken from nodejs client streaming.

However I need help on how to write test spec if we are expected to fail on some browsers.

[timostamm]

@oott123, let me wrap up #673 first, I will take a look then.

[timostamm]

Apologies that this hasn't gotten attention yet, @oott123. Hope we can help push this across soon.

[timostamm]

I got pulled into other directions, apologies.

Unfortunately, Safari and Firefox still do not implement the feature. We have a more comprehensive test suite now that allows us to more comprehensively test client streams, but it doesn't run on old browsers yet (see #810).

So this feature is blocked on two sides: Our test coverage isn't sufficient yet, and browsers don't support it across the board. This means that we cannot provide this feature yet.

Once Safari and Firefox have caught up, we can revisit.