An optional interface FileInfoNames has been added.
If the parameter fi of FileInfoHeader implements the interface
the Gname/Uname of the return value Header
are provided by the method of the interface.
Also added testing.
Fixes#50102
Change-Id: I47976e238eb20ed43113b060e4f83a14ae49493e
GitHub-Last-Rev: a213613c79e150d52a2f5c84dca7a49fe123fa40
GitHub-Pull-Request: golang/go#65273
Reviewed-on: https://go-review.googlesource.com/c/go/+/558355
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
