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.
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