fix(ngrid): do not auto-clear context on source changing

BREAKING CHANGE
Until now, everytime the `onSourceChanging` event was fired, the context cache was automatically cleared.
This event is fired when `refresh()` is called and automatically clearing the context is bad, if the user wants to clear the cache he should do it by calling `grid.contextApi.clear()`.

The cache will still auto clear when the datasource is changed and when render data is changing without a primary key

To revert back to the old behavior you can set `clearContextOnSourceChanging` to true on the global configuration value for table.

libs/ngrid/src/lib/grid/ngrid.component.ts

@@ -512,9 +512,12 @@ export class PblNgridComponent<T = any> implements AfterContentInit, AfterViewIn
         // This is required because `onRenderDataChanging` is fired async, just before the data is emitted.
         // Its not enough to clear the context when `setDataSource` is called, we also need to handle `refresh` calls which will not
         // trigger this method.
-        value.onSourceChanging.pipe(unrx(this, value))
+        value.onSourceChanging
+          .pipe(unrx(this, value))
           .subscribe( () => {
-            this._extApi.contextApi.clear();
+            if (this.config.get('table').clearContextOnSourceChanging) {
+              this._extApi.contextApi.clear();
+            }
           });
 
         // Run CD, scheduled as a micro-task, after each rendering

libs/ngrid/src/lib/grid/services/config.ts

@@ -7,13 +7,15 @@ export interface PblNgridConfig {
     showHeader?: boolean;
     showFooter?: boolean;
     noFiller?: boolean;
+    clearContextOnSourceChanging?: boolean;
   }
 }
 
 const DEFAULT_TABLE_CONFIG: PblNgridConfig['table'] = {
   showHeader: true,
   showFooter: false,
   noFiller: false,
+  clearContextOnSourceChanging: false,
 };
 
 export const PEB_NGRID_CONFIG = new InjectionToken<PblNgridConfig>('PEB_NGRID_CONFIG');