GitLab

* Adds StartOffset to ReaderObjectAttrs, which for range
requests is the value from which the read began
* Updates the range reader integration tests to also test
negative offsets aka offsets from the end of the file
* Adds integration tests for the various styles of reading
the same last n bytes from a file
* Adds example tests to show the various styles of using
(*ObjectHandle).NewRangeReader
* Replaces CL https://code-review.googlesource.com/c/gocloud/+/44132
for which the original author @brimworks no longer has
Gerrit access due to job changes

Updates #1540.

Change-Id: I56b7777cc46e3c501593e92e27a7c083a729fed5
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/44610

Reviewed-by: Frank Natividad <[email protected]>
Reviewed-by: kokoro <[email protected]>