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>