Airflow 2.0.1 Security, configure permission on button Import Variables

[pushkinsk44]

Description

Add the ability to set permission for the "Import Variables" button

Use case / motivation

For example, in production environment, some users need to look at variables
but they shouldn't have the right to change anything about them
if you add permission "can read on Variables" to the Viewer role, then the Import Variables button is active for users and using it they can change variables

[boring-cyborg]

Thanks for opening your first issue here! Be sure to follow the issue template!

[kaxil]

cc @jhtimmins

[zoomzoomTnT]

cc @jhtimmins this issue still is in 2.0.2