add an endpoint for doing SQL selects (#641)

* add an endpoint for doing SQL selects

This adds an endpoint for doing SQL selects directly on a Grist document. Other kinds of statements are not supported. There is a default timeout of a second on queries.

This follows loosely an API design by Alex Hall.

Co-authored-by: jarek <jaroslaw.sadzinski@gmail.com>

app/server/lib/AppSettings.ts

@@ -96,6 +96,17 @@ export class AppSettings {
     return result;
   }
 
+  /**
+   * As for readInt() but fail if nothing was found.
+   */
+  public requireInt(query: AppSettingQuery): number {
+    const result = this.readInt(query);
+    if (result === undefined) {
+      throw new Error(`missing environment variable: ${query.envVar}`);
+    }
+    return result;
+  }
+
   /**
    * As for read() but type (and store, and report) the result as
    * a boolean.
@@ -107,6 +118,17 @@ export class AppSettings {
     return result;
   }
 
+  /**
+   * As for read() but type (and store, and report) the result as
+   * an integer (well, a number).
+   */
+  public readInt(query: AppSettingQuery): number|undefined {
+    this.readString(query);
+    const result = this.getAsInt();
+    this._value = result;
+    return result;
+  }
+
   /* set this setting 'manually' */
   public set(value: JSONValue): void {
     this._value = value;