support public versioned S3 access

Makes git annex whereis display the versionId urls.

And, when a s3 remote is enabled without creds, git-annex will use the
versionId urls to access its contents.

This commit was sponsored by Fernando Jimenez on Patreon.

doc/todo/support_public_versioned_S3_access.mdwn

@@ -5,3 +5,5 @@ does not yet support that, nor does whereis show the urls.
 Should not be super hard to add, but it involves converting `getpublicurl`
 into an Annex action and distinguishing between different uses of it,
 some of which work with this and some don't. --[[Joey]]
+
+> [[done]] --[[Joey]]