(core) External requests

Summary:
Adds a Python function `REQUEST` which makes an HTTP GET request. Behind the scenes it:

- Raises a special exception to stop trying to evaluate the current cell and just keep the existing value.
- Notes the request arguments which will be returned by `apply_user_actions`.
- Makes the actual request in NodeJS, which sends back the raw response data in a new action `RespondToRequests` which reevaluates the cell(s) that made the request.
- Wraps the response data in a class which mimics the `Response` class of the `requests` library.

In certain cases, this asynchronous flow doesn't work and the sandbox will instead synchronously call an exported JS method:

- When reevaluating a single cell to get a formula error, the request is made synchronously.
- When a formula makes multiple requests, the earlier responses are retrieved synchronously from files which store responses as long as needed to complete evaluating formulas. See https://grist.slack.com/archives/CL1LQ8AT0/p1653399747810139

Test Plan: Added Python and nbrowser tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz, dsagal

Differential Revision: https://phab.getgrist.com/D3429

app/server/lib/Sharing.ts

@@ -6,7 +6,7 @@ import {
   LocalActionBundle,
   UserActionBundle
 } from 'app/common/ActionBundle';
-import {DocAction, getNumRows, UserAction} from 'app/common/DocActions';
+import {CALCULATING_USER_ACTIONS, DocAction, getNumRows, UserAction} from 'app/common/DocActions';
 import {allToken} from 'app/common/sharing';
 import * as log from 'app/server/lib/log';
 import {LogMethods} from "app/server/lib/LogMethods";
@@ -215,8 +215,7 @@ export class Sharing {
 
     try {
 
-      const isCalculate = (userActions.length === 1 &&
-                           (userActions[0][0] === 'Calculate' || userActions[0][0] === 'UpdateCurrentTime'));
+      const isCalculate = (userActions.length === 1 && CALCULATING_USER_ACTIONS.has(userActions[0][0] as string));
       // `internal` is true if users shouldn't be able to undo the actions. Applies to:
       // - Calculate/UpdateCurrentTime because it's not considered as performed by a particular client.
       // - Adding attachment metadata when uploading attachments,