(core) Draft version of AI assistant

Summary:
The feature is behind a flag GRIST_FORMULA_ASSISTANT (must be "true"). But can be enabled in the
developer console by invoking GRIST_FORMULA_ASSISTANT.set(true).

Keys can be overriden in the document settings page.

Test Plan: For now just a stub test that checks if this feature is disabled by default.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3815

app/common/ActiveDocAPI.ts

@@ -1,4 +1,5 @@
 import {ActionGroup} from 'app/common/ActionGroup';
+import {Prompt, Suggestion} from 'app/common/AssistancePrompts';
 import {BulkAddRecord, CellValue, TableDataAction, UserAction} from 'app/common/DocActions';
 import {FormulaProperties} from 'app/common/GranularAccessClause';
 import {UIRowId} from 'app/common/UIRowId';
@@ -322,7 +323,7 @@ export interface ActiveDocAPI {
   /**
    * Generates a formula code based on the AI suggestions, it also modifies the column and sets it type to a formula.
    */
-  getAssistance(tableId: string, colId: string, description: string): Promise<void>;
+  getAssistance(userPrompt: Prompt): Promise<Suggestion>;
 
   /**
    * Fetch content at a url.

app/common/gristUrls.ts

@@ -583,6 +583,9 @@ export interface GristLoadConfig {
   // TODO: remove when comments will be released.
   featureComments?: boolean;
 
+  // TODO: remove once released.
+  featureFormulaAssistant?: boolean;
+
   // Email address of the support user.
   supportEmail?: string;