Add filter for the WXR export filename

[alfreddatakillen]

Hi! I am working in a team that is using Git for storing WXR export files from lots of WordPress multisite blogs. We want our own filename naming conventions for those WXR export files (blog ID number, and not having a date in the filename, since Git is doing versioning already). However, there are just too many human errors when manually renaming filenames. So, to facilitate such teamwork, we believe there should be a filter for the export file filename.

[alfreddatakillen]

Adding a export_filename filter. And a unit test.

[alfreddatakillen]

Uhmm.. I'm a newcomer here. I thought the diff file would get the ticket ID in the filename automagically. Uploading the same file again, with ticket ID filename.

[alfreddatakillen]

Adding a export_filename filter. And a unit test.

[MikeHansenMe]

Same patch fixed some spaces

[wonderboymusic]

In 34327:

Export: Add a filer, 'export_wp_filename' to change the filename of the file to download when exporting.

Props MikeHansenMe, wonderboymusic.
Fixes #29500.