This adds the ability to "peek" at the value of a SyncValue, so that
it's possible to observe a value without computing this.
Updates tailscale/corp#17122
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I06f88c22a1f7ffcbc7ff82946335356bb0ef4622