feat: add json file transport

src/Transport/FileTransportInterface.php

@@ -0,0 +1,16 @@
+<?php
+/**
+ * File package transport boundary.
+ *
+ * @package WPContentSync
+ */
+
+namespace WPContentSync\Transport;
+
+use WPContentSync\Package\ContentPackage;
+
+interface FileTransportInterface {
+	public function export( ContentPackage $package ): string;
+
+	public function import( string $contents ): ContentPackage;
+}

src/Transport/JsonFileTransport.php

@@ -0,0 +1,45 @@
+<?php
+/**
+ * JSON file transport implementation.
+ *
+ * @package WPContentSync
+ */
+
+namespace WPContentSync\Transport;
+
+use WPContentSync\Package\ContentPackage;
+use WPContentSync\Package\PackageValidator;
+
+final class JsonFileTransport implements FileTransportInterface {
+	private PackageValidator $validator;
+
+	public function __construct( PackageValidator $validator ) {
+		$this->validator = $validator;
+	}
+
+	public function export( ContentPackage $package ): string {
+		$json = wp_json_encode( $package->toArray(), JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES );
+
+		if ( false === $json ) {
+			throw new \RuntimeException( 'Unable to encode content package JSON.' );
+		}
+
+		return $json;
+	}
+
+	public function import( string $contents ): ContentPackage {
+		$decoded = json_decode( $contents, true );
+
+		if ( ! is_array( $decoded ) ) {
+			throw new \InvalidArgumentException( 'The selected file is not valid JSON.' );
+		}
+
+		$result = $this->validator->validate( $decoded );
+
+		if ( ! $result->isValid() ) {
+			throw new \InvalidArgumentException( implode( ' ', $result->errors() ) );
+		}
+
+		return ContentPackage::fromArray( $decoded );
+	}
+}