(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/ISandbox.ts

@@ -1,4 +1,5 @@
 import * as log from 'app/server/lib/log';
+import {ISandboxOptions} from 'app/server/lib/NSandbox';
 
 /**
  * Starting to whittle down the options used when creating a sandbox, to leave more
@@ -17,6 +18,8 @@ export interface ISandboxCreationOptions {
   importMount?: string;  // if defined, make this path available read-only as "/importdir"
 
   preferredPythonVersion?: '2' | '3';
+
+  sandboxOptions?: Partial<ISandboxOptions>;
 }
 
 export interface ISandbox {