Add support for a header which causes WordPress.org not to offer an update for an item - #950

This PR introduces a header that the WordPress.org plugins api will ignore for the purposes of plugin updates.
This PR doesn't touch themes, as this is proof-of-concept.

The Update URI field should:

• Be a URI, eg: https://wordpress.org/plugins/example-plugin, or https://dd32.id.au/plugins/example-plugin
• Have a unique hostname
• Must pass through esc_url_raw() so custom scheme's are out. github://someone/something is not valid.
• https://some-url-that-will-never-exist-at-all-ever or my-custom-plugin-name are valid.
• Update URI: false is valid, and unless code is handling the 'false' hostname, the plugin will never get an update notification.

If present, the WordPress.org plugins API will currently not return a result for it, UNLESS, it matches the following format:

• w.org/plugin/{$slug}
• https://wordpress.org/plugins/{$slug}/
  (This is alpha, so it's been added to the updates API but will be removed if this PR doesn't end up in WordPress.)

Included, is a filter which 3rd party plugins can use to offer updates for a given hostname (Yes, it's tied to a hostname), and they can return their plugin data blob and it should just work natively.

There's a few other small changes around WordPress not expecting that the slug field is always set, but that's the only real change to the rest of WordPress needed.

Included is also an example plugin, whose Name + Author indicate that it's the WordPress.org Hello Dolly plugin.
When installed on...

• WordPress 5.6: Offers to update to the Hello Dolly plugin.
• This PR, plugin deactivated: No update offered, as no plugin is handling the updates for it.
• This PR, plugin activated: Offers to update the plugin using the hard-coded details from the plugin (This should be pulled from an API)

Trac ticket: https://core.trac.wordpress.org/ticket/14179

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.