(core) New type conversion in the backend

Summary: This is https://phab.getgrist.com/D3205 plus some changes (https://github.com/dsagal/grist/compare/type-convert...type-convert-server?expand=1) that move the conversion process to the backend. A new user action ConvertFromColumn uses `call_external` so that the data engine can delegate back to ActiveDoc. Code for creating formatters and parsers is significantly refactored so that most of the logic is in `common` and can be used in different ways.

Test Plan: The original diff adds plenty of tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3240

app/server/lib/NSandbox.ts

@@ -33,7 +33,7 @@ type SandboxMethod = (...args: any[]) => any;
  * started by setting `useGristEntrypoint` (the only exception is
  * in tests) which runs grist/main.py.
  */
-interface ISandboxOptions {
+export interface ISandboxOptions {
   command?: string;       // External program or container to call to run the sandbox.
   args: string[];         // The arguments to pass to the python process.
 
@@ -404,6 +404,7 @@ export class NSandboxCreator implements ISandboxCreator {
       preferredPythonVersion: this._preferredPythonVersion || options.preferredPythonVersion,
       useGristEntrypoint: true,
       importDir: options.importMount,
+      ...options.sandboxOptions,
     };
     return new NSandbox(translatedOptions, spawners[this._flavor]);
   }