Implement sending middleware.
Sending middleware is useful for adding tracing, progress tokens,
and so on.
Change-Id: I0acab2550f0212d549c4adbcf1ebf425a9486df6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676355
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
