Plugins: Add support for Update URI header.

This allows third-party plugins to avoid accidentally being overwritten with an update of a plugin of a similar name from the WordPress.org Plugin Directory.

Additionally, introduce the `update_plugins_{$hostname}` filter, which third-party plugins can use to offer updates for a given hostname.

If set, the `Update URI` header field should be a URI and have a unique hostname.

Some examples include:

* `https://wordpress.org/plugins/example-plugin/`
* `https://example.com/my-plugin/`
* `my-custom-plugin-name`

`Update URI: false` also works, and unless there is code handling the `false` hostname, the plugin will never get an update notification.

If the header is present, the WordPress.org API will currently only return updates for the plugin if it matches the following format:

* `https://wordpress.org/plugins/{$slug}/`
* `w.org/plugin/{$slug}`

If the header has any other value, the API will not return a result and will ignore the plugin for update purposes.

Props dd32, DavidAnderson, meloniq, markjaquith, DrewAPicture, mweichert, design_dolphin, filosofo, sean212, nhuja, JeroenReumkens, infolu, dingdang, joyously, earnjam, williampatton, grapplerulrich, markparnell, apedog, afragen, miqrogroove, rmccue, crazycoders, jdgrimes, damonganto, joostdevalk, jorbin, georgestephanis, khromov, GeekStreetWP, jb510, Rarst, juliobox, Ipstenu, mikejolley, Otto42, gMagicScott, TJNowell, GaryJ, knutsp, mordauk, nvartolomei, aspexi, chriscct7, benoitchantre, ryno267, lev0, gregorlove, dougwollison, SergeyBiryukov.
See #14179, #23318, #32101.

git-svn-id: https://develop.svn.wordpress.org/trunk@50921 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/plugin.php

@@ -44,6 +44,7 @@
  *
  * @since 1.5.0
  * @since 5.3.0 Added support for `Requires at least` and `Requires PHP` headers.
+ * @since 5.8.0 Added support for `Update URI` header.
  *
  * @param string $plugin_file Absolute path to the main plugin file.
  * @param bool   $markup      Optional. If the returned data should have HTML markup applied.
@@ -63,6 +64,7 @@
  *     @type bool   $Network     Whether the plugin can only be activated network-wide.
  *     @type string $RequiresWP  Minimum required version of WordPress.
  *     @type string $RequiresPHP Minimum required version of PHP.
+ *     @type string $UpdateURI   ID of the plugin for update purposes, should be a URI.
  * }
  */
 function get_plugin_data( $plugin_file, $markup = true, $translate = true ) {
@@ -79,6 +81,7 @@ function get_plugin_data( $plugin_file, $markup = true, $translate = true ) {
 		'Network'     => 'Network',
 		'RequiresWP'  => 'Requires at least',
 		'RequiresPHP' => 'Requires PHP',
+		'UpdateURI'   => 'Update URI',
 		// Site Wide Only is deprecated in favor of Network.
 		'_sitewide'   => 'Site Wide Only',
 	);