proposal: x/net/http2: add api to send data frame one by one during invocation

[LaurenceLiZhixin]

What version of Go are you using (go version)?

$ go version 
go version go1.15.5 darwin/amd64

Does this issue reproduce with the latest release?

None

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"

What did you do?

What did you expect to see?

I want to use x/net/http2 pkg to impl grpc stream-rpc invocation, but I found that it only supports invocation with header + bodyframe, not header + body + body + body ... be along with the flash of stream.

What did you see instead?

[ianlancetaylor]

Can you suggest an API? Thanks.

[networkimprov]

cc @fraenkel