Before this commit it was not possible to override the the ContentType
and ETag properties.

Since these properties aren't directly read from the os.FileInfo
objects returned by the FileSystem it seems reasonable that the user
might have a different policy for computing them.

For instance the underlying FileSystem may already know the
ContentType or want to use an MD5 Hash for the ETag.

This commit introduces two new optional interfaces ETager and
ContentTyper which, when defined on the os.FileInfo objects
returned by the FileSystem methods, allows the user of this library to
override the ETag and ContentType generation.

Fixes golang/go#22577

Change-Id: Ib42e126db3fcc0a93463e61db85fde59be85cca5
Reviewed-on: https://go-review.googlesource.com/109217Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>