Allow TOTP keys to be filterable

Since the discussion in WordPress#389 seems to be on-going, the following should allow users of the plugin to decide how they would like to approach the encryption part.

Perhaps we can follow up with solutions after they've been implemented and debate further?

providers/class-two-factor-totp.php

@@ -420,7 +420,8 @@ public function user_two_factor_options( $user ) {
 	 * @return string
 	 */
 	public function get_user_totp_key( $user_id ) {
-		return (string) get_user_meta( $user_id, self::SECRET_META_KEY, true );
+		$meta = (string) get_user_meta( $user_id, self::SECRET_META_KEY, true );
+		return apply_filters( 'two_factor_totp_get_user_totp_key', $meta, $user_id );
 	}
 
 	/**
@@ -432,6 +433,7 @@ public function get_user_totp_key( $user_id ) {
 	 * @return boolean If the key was stored successfully.
 	 */
 	public function set_user_totp_key( $user_id, $key ) {
+		$key = apply_filters( 'two_factor_totp_set_user_totp_key', $key, $user_id );
 		return update_user_meta( $user_id, self::SECRET_META_KEY, $key );
 	}