Themes: Add a hook to filter theme header image URL.

This changeset introduces the `get_header_image` filter, which can be used to modify header image URL returned by `get_header_image()`, in themes that support the Header Image feature. 

Props hztyfoon, audrasjb, mukesh27, SergeyBiryukov, costdev.
Fixes #56180.


git-svn-id: https://develop.svn.wordpress.org/trunk@53741 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1179,6 +1179,20 @@ function get_header_image() {
 		$url = get_random_header_image();
 	}
 
+	/**
+	 * Filters the header image URL.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @param string $url Header image URL.
+	 */
+	$url = apply_filters( 'get_header_image', $url );
+
+	if ( ! is_string( $url ) ) {
+		return false;
+	}
+
+	$url = trim( $url );
 	return sanitize_url( set_url_scheme( $url ) );
 }