Add JSON format to `mail:account:export`

[WilliamDEdwards]

Is your feature request related to a problem? Please describe.

php occ mail:account:delete takes a user-id. This user ID can only be retrieved with php occ mail:account:export, but it doesn't support JSON output, which makes automatically deleting mail accounts nearly impossible.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response