chore(deps): vendor plugin-update-checker v5.6

Source: https://github.com/YahnisElsts/plugin-update-checker/releases/tag/v5.6
Used in 1.2.0 for self-hosted updates from Gitea (Generic JSON mode).

includes/lib/plugin-update-checker/Puc/v5p6/Vcs/BaseChecker.php

@@ -0,0 +1,29 @@
+<?php
+namespace YahnisElsts\PluginUpdateChecker\v5p6\Vcs;
+
+if ( !interface_exists(BaseChecker::class, false) ):
+
+	interface BaseChecker {
+		/**
+		 * Set the repository branch to use for updates. Defaults to 'master'.
+		 *
+		 * @param string $branch
+		 * @return $this
+		 */
+		public function setBranch($branch);
+
+		/**
+		 * Set authentication credentials.
+		 *
+		 * @param array|string $credentials
+		 * @return $this
+		 */
+		public function setAuthentication($credentials);
+
+		/**
+		 * @return Api
+		 */
+		public function getVcsApi();
+	}
+
+endif;