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.\n\nBody parameters:\n- `sites` (integer array, required): exact ids, 1 to 200.\n- `is_hidden` (boolean, required): `true` hides the update notice, `false` shows it again. The requested state, never a flip.\n\n200 with a meta-only document: `meta.status` (`applied`|`skipped`), `meta.updated` (`{site_id, site_item_id}` per changed item) and `meta.skipped`.\n\n- **422**: `sites` missing/empty/over 200/unknown id; `is_hidden` missing or not boolean.\n- **404**: `{component}` does not exist or is not reachable from the token's organization.\n- **403**: a read-only token.