(core) Porting back AI formula backend

Summary: This is a backend part for the formula AI.

Test Plan: New tests

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: cyprien

Differential Revision: https://phab.getgrist.com/D3786

app/common/ActiveDocAPI.ts

@@ -318,6 +318,11 @@ export interface ActiveDocAPI {
    */
   getFormulaError(tableId: string, colId: string, rowId: number): Promise<CellValue>;
 
+  /**
+   * 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>;
+
   /**
    * Fetch content at a url.
    */