Feat add get export form to export mixin

[KamilRizatdinov]

Problem

What problem have you solved?

You can see how hard it is to use the custom export form. You have to override the whole export_action() method.
https://stackoverflow.com/a/67653671/17465982 referes to this problem as well

Solution

How did you solve the problem?

Introduced the get_export_form() method on ExportMixin as an analogue to get_import_form() from ImportMixin

Acceptance Criteria

Have you written tests? Have you included screenshots of your changes if applicable?
Did you document your changes?

Yes, I've written the test cases to ensure that both the base functionality and functionality work
I used the docstrings for the get_export_form() method, think that's a sufficient documentation

[matthewhegarty]

Thanks for this. Please could you add the following to the changelog:

add get_export_form() to ExportMixin (#1409)

Please could you also squash the commits into one and push.
Feel free to add your name to AUTHORS.

[KamilRizatdinov]

Thanks for this. Please could you add the following to the changelog:

add get_export_form() to ExportMixin (#1409)

Please could you also squash the commits into one and push. Feel free to add your name to AUTHORS.

All done!

[coveralls]

Coverage increased (+0.003%) to 98.394% when pulling 227de9c on KamilRizatdinov:feat_add_get_export_form_to_export_mixin into 8d6e46f on django-import-export:main.