(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

sandbox/grist/main.py

@@ -68,10 +68,13 @@ def run(sandbox):
   @export
   def apply_user_actions(action_reprs, user=None):
     action_group = eng.apply_user_actions([useractions.from_repr(u) for u in action_reprs], user)
-    return dict(
+    result = dict(
       rowCount=eng.count_rows(),
       **eng.acl_split(action_group).to_json_obj()
     )
+    if action_group.requests:
+      result["requests"] = action_group.requests
+    return result
 
   @export
   def fetch_table(table_id, formulas=True, query=None):