appservice: keep the crypto method set on the wrapped state store

The lazy store embedded appservice.StateStore, but the client's store is
also type-asserted to crypto.StateStore -- a wider interface -- and
NewCryptoHelper refuses to start when that assertion fails. Embedding the
narrower interface dropped GetEncryptionEvent and FindSharedRooms from the
concrete type, so the bot died on startup with "the client state store
must implement crypto.StateStore".

Embed both, assert both at compile time, and resolve GetEncryptionEvent
lazily like IsEncrypted since the crypto machine reads megolm rotation
settings through it.

internal/bot/appservice.go

@@ -110,7 +110,11 @@ func newAppserviceSession(cfg Config) (*Session, error) {
 	// use, since there is no /sync to backfill it. Must be installed before the
 	// first BotClient() call: makeClient copies as.StateStore into the client, and
 	// caches the client.
-	store := newLazyStateStore(as.StateStore)
+	inner, ok := as.StateStore.(innerStateStore)
+	if !ok {
+		return nil, fmt.Errorf("appservice state store %T does not implement crypto.StateStore", as.StateStore)
+	}
+	store := newLazyStateStore(inner)
 	as.StateStore = store
 
 	client := as.BotClient() // as_token auth + SetAppServiceUserID (?user_id=) assertion