LocalDataSource - add option to find by predicate

like:
findBy(predicate:any): Promise;

this can be the code:

```
LocalDataSource.prototype.findBy = function (predicate){
if (typeof predicate !== 'function') {
throw new TypeError('predicate must be a function');
}
const found = this.data.find(predicate);
if (found) {
return Promise.resolve(found);
}

return Promise.reject(new Error('Element was not found in the dataset'));
}

```