Prepare HTMX response headers

Closes #3523727

core/lib/Drupal/Core/Render/Hypermedia/HtmxHeaderInterface.php

+<?php
+
+namespace Drupal\Core\Render\Hypermedia;
+
+/**
+ * An interface for classes that manage HTMX Headers.
+ *
+ * @extends \IteratorAggregate<string, list<string|null>>
+ */
+interface HtmxHeaderInterface extends \IteratorAggregate, \Countable, \Stringable {
+
+  /**
+   * Checks if the storage has a header with the given name.
+   *
+   * @param string $name
+   *   The name of the header to check for.
+   *
+   * @return bool
+   *   Returns TRUE if the header exists, or FALSE otherwise.
+   */
+  public function hasHeader(string $name): bool;
+
+  /**
+   * Returns all storage elements as a Drupal 'http_header' array.
+   *
+   * @return mixed[]
+   *   An associative array of headers.
+   */
+  public function toArray(): array;
+
+  /**
+   * Merges an HtmxHeaderInterface object into the current storage.
+   *
+   * @param HtmxHeaderInterface $headers
+   *   The Attribute object to merge.
+   *
+   * @return HtmxHeaderInterface
+   *   A combined header collection.
+   */
+  public function merge(HtmxHeaderInterface $headers): HtmxHeaderInterface;
+
+}