bbPress.org

trying to get the feature image funtionality on front end so user can have the option to upload image about the Topic

This sounds like custom development , you might need a developer to do this type of thing.

get the feature image on Widget “Forum list” Default widget of BbPress plugin

You would need to create a new “Forums list” widget mostly based off the original bbPress default widget.

Then use get_the_post_thumbnail to render your featured images

https://codex.wordpress.org/Function_Reference/get_the_post_thumbnail
https://codex.wordpress.org/Post_Thumbnails

i have to create a new widget for own customization work and where i have to place that file

If you know what you are doing , you can place it in a child themes functions.php , or just place it in a new file like says widgets.php to better organize your content then include the file using something like this in your functions.php

require get_template_directory() . '/inc/widgets.php';

You can also place the code in a custom plugin too.

You shouldnt place any customizations in the bbPress plugins folder, because you will lose the changes next update.

And You dont need to place the customized widget funcitons in the bbpress folder in your child theme either.