tailcfg: report StateEncrypted in Hostinfo (#16434)

Report whether the client is configured with state encryption (which
varies by platform and can be optional on some). Wire it up to
`--encrypt-state` in tailscaled, which is set for Linux/Windows, and set
defaults for other platforms. Macsys will also report this if full
Keychain migration is done.

Updates #15830

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/store.go

@@ -113,3 +113,9 @@ func ReadStoreInt(store StateStore, id StateKey) (int64, error) {
 func PutStoreInt(store StateStore, id StateKey, val int64) error {
 	return WriteState(store, id, fmt.Appendf(nil, "%d", val))
 }
+
+// EncryptedStateStore is a marker interface implemented by StateStores that
+// encrypt data at rest.
+type EncryptedStateStore interface {
+	stateStoreIsEncrypted()
+}