allow to export data from multiget

[georgehrke]

@skjnldsv You can't trigger downloads from an XMLHttpRequest, so you still need to do some browser magic.

Requires nextcloud/server#12004

const client = await new cdav.default({rootUrl: 'http://all.local/nextcloud/remote.php/dav'}, xhrProvider).connect({enableCardDAV: true, enableCalDAV: true});
const addressbooks = await client.addressBookHomes[0].findAllAddressBooks();
const contacts = await addressbooks[0].findAll();
const urls = contacts.slice(2, 8).map((c) => c.url);
const data = await addressbooks[0].addressbookMultigetExport(urls);
const blob = new Blob([data.body], {type: "text/vcard"});
const url = URL.createObjectURL(blob);
const link = document.createElement('a');
const dispositionHeader = data.xhr.getResponseHeader('Content-Disposition');
const filename = /filename[^;=\n]*=((['"]).*?\2|[^;\n]*)/.exec(dispositionHeader)[1].replace(/['"]/g, '');
link.href = url;
link.download = filename;
link.click()