when will Apple put a USB on the iPad to plugin an external drive

They already have.

The newest iPads have a USB-C port and with the new iPadOS 13 to be released in the coming months they will support USB drives.

See here: iPadOS Preview - Apple

Read all the new features coming this fall with iPadOS: iPadOS Preview - Features - Apple

This will only be possible for you if you have an iPad which is capable of using iPadOS.