Update component visibility

Hides or shows one plugin, theme or WordPress core update notice on the selection: a direct `is_hidden` write on its site items, no Site Action and no connectivity check. Body parameters: - `sites` (integer array, required): exact ids, 1 to 200. - `is_hidden` (boolean, required): `true` hides the update notice, `false` shows it again. The requested state, never a flip. 200 with a meta-only document: `meta.status` (`applied`|`skipped`), `meta.updated` (`{site_id, site_name, site_item_id}` per changed item) and `meta.skipped` (`{site_id, site_name, site_item_id, reason_code, message}` per refused website; `site_item_id` is null there, `site_name` is null for `PERMISSION_DENIED`; `NOT_FOUND` when the component is not installed on the website, `ALREADY_APPLIED` when it is already in the requested state). - **422**: `sites` missing/empty/over 200/unknown id; `is_hidden` missing or not boolean. - **404**: `{component}` does not exist or is not reachable from the token's organization. - **403**: a read-only token.