ipn/ipnlocal: add c2n method to get SSH username candidates

For control to fetch a list of Tailscale SSH username candidates to
filter against the Tailnet's SSH policy to present some valid
candidates to a user.

Updates #3802
Updates tailscale/corp#7007

Change-Id: I3dce57b7a35e66891d5e5572e13ae6ef3c898498
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/ssh_stub.go

@@ -6,6 +6,16 @@
 
 package ipnlocal
 
+import (
+	"errors"
+
+	"tailscale.com/tailcfg"
+)
+
 func (b *LocalBackend) getSSHHostKeyPublicStrings() []string {
 	return nil
 }
+
+func (b *LocalBackend) getSSHUsernames(*tailcfg.C2NSSHUsernamesRequest) (*tailcfg.C2NSSHUsernamesResponse, error) {
+	return nil, errors.New("not implemented")
+}