upspin: add SignedName
upspin: add SignedName

For supporting snapshotting and other dynamic DirServers.

Change-Id: Ic7e6800c750c0b0c51472c80c07fc6cfce878451
Reviewed-on: https://upspin-review.googlesource.com/5100
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>
Reviewed-by: Eric Grosse <f6ae9ca1078fda92fd7c2c20411c29bc6711159e@google.com>
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin