(core) Improve object serialization, to help get RECORD data to Custom Widgets.

Summary:
- Change RECORD's dates_as_str default to False.
- Reimplement objtype encode_object/decode_object with less machinery.
- Implement encoding of dicts (with string keys).
- Make lists and dicts encode values recursively.
- Implement encoding/decoding in the client
- Decode automatically in plugins' fetchSelectedTable/Record, with an option to skip.

Test Plan: Tested manually, not sure what tests may be affected yet.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2593

app/plugin/GristData.ts

@@ -1,4 +1,4 @@
-export type CellValue = number|string|boolean|null|[string, any?];
+export type CellValue = number|string|boolean|null|[string, ...unknown[]];
 
 export interface RowRecord {
   id: number;