(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/common/DocActions.ts

@@ -144,6 +144,9 @@ export interface TableRecordValue {
 
 export type UserAction = Array<string|number|object|boolean|null|undefined>;
 
+// Actions that trigger formula calculations in the data engine
+export const CALCULATING_USER_ACTIONS = new Set(['Calculate', 'UpdateCurrentTime', 'RespondToRequests']);
+
 /**
  * Gives a description for an action which involves setting values to a selection.
  * @param {Array} action - The (Bulk)AddRecord/(Bulk)UpdateRecord action to describe.