(core) Add /records endpoint to DocApi with GET, POST, and PATCH

Summary:
Applies simple data transformations to the existing /data API.

Mimics the Airtable API. Designed in https://grist.quip.com/RZh9AEbPaj8x/Doc-API#FZfACAAZ9a0

Haven't done deletion because it seems like less of a priority and also not fully designed.

Test Plan: Added basic server tests similar to the /data tests. Haven't tested edge cases like bad input.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D2974

app/common/DocActions.ts

@@ -117,6 +117,18 @@ export interface TableColValues {
   [colId: string]: CellValue[];
 }
 
+// Multiple records in record-oriented format
+export interface TableRecordValues {
+  records: TableRecordValue[];
+}
+
+export interface TableRecordValue {
+  id: number;
+  fields: {
+    [colId: string]: CellValue
+  };
+}
+
 // Both UserActions and DocActions are represented as [ActionName, ...actionArgs].
 // TODO I think it's better to represent DocAction as a Buffer containing the marshalled action.