src/vs/platform/userDataSync/common/userDataSyncStoreService.ts/UserDataSyncStoreClient/writeResource
class UserDataSyncStoreClient:	async writeResource(resource: ServerResource, data: string, ref: string | null, collection?: string, headers: IHeaders = {}): Promise<string> {
		if (!this.userDataSyncStoreUrl) {
			throw new Error('No settings sync store url configured.');
		}

		const url = this.getResourceUrl(this.userDataSyncStoreUrl, collection, resource).toString();
		headers = { ...headers };
		headers['Content-Type'] = Mimes.text;
		if (ref) {
			headers['If-Match'] = ref;
		}

		const context = await this.request(url, { type: 'POST', data, headers }, [], CancellationToken.None);

		const newRef = context.res.headers['etag'];
		if (!newRef) {
			throw new UserDataSyncStoreError('Server did not return the ref', url, UserDataSyncErrorCode.NoRef, context.res.statusCode, context.res.headers[HEADER_OPERATION_ID]);
		}
		return newRef;
	}