feat: Remove old users from export ownership

[Vitor-Avila]

This PR implements some changes to the export-ownership command:

• Adds an --asset-type filter to the command, which allows to export ownership only for dashboard, dataset or chart. Multiple can be passed.
• In case an asset ha an owner that's no longer in the team, it logs the exception.
• Adds a new --exclude-old-users flag, to automatically remove these old users from the generated file. This way, a new import would clean up the configurations.

[Vitor-Avila]

Moved this to the command layer to avoid calling it once for every asset type.

[betodealmeida]

Nice!

Do we need/want to have an option to assign those assets from excluded users to the admin or some other user?

[Vitor-Avila]

Do we need/want to have an option to assign those assets from excluded users to the admin or some other user?

I thought this was a great idea, but then it got me thinking that admins are already able to manage ownership without being owners, and having a single account to replace all old users could be too limited. Ideally admins would run this script to migrate ownership before offboarding a user, but I think the longer term solution is to actually accept groups in the ownership configuration.