Add --dry-run option for add-missing-* cmd

[artonge]

Output example after updating from 21 to master:

Check indices of the share table.
Check indices of the filecache table.
Check indices of the twofactor_providers table.
Check indices of the login_flow_v2 table.
Check indices of the whats_new table.
Check indices of the cards table.
Check indices of the cards_properties table.
Check indices of the calendarobjects_props table.
Check indices of the schedulingobjects table.
Check indices of the oc_properties table.
Check indices of the oc_jobs table.
Adding job_lastcheck_reserved index to the oc_jobs table, this can take some time...
CREATE INDEX job_lastcheck_reserved ON oc_jobs (last_checked, reserved_at);

oc_properties table updated successfully.
Check indices of the oc_direct_edit table.
Adding direct_edit_timestamp index to the oc_direct_edit table, this can take some time...
CREATE INDEX direct_edit_timestamp ON oc_direct_edit (timestamp);

CREATE INDEX job_lastcheck_reserved ON oc_jobs (last_checked, reserved_at);

oc_direct_edit table updated successfully.

PR for documentation: nextcloud/documentation#8084

Fix #29959
Fix https://github.com/nextcloud-gmbh/server/issues/141

[ChristophWurst]

this works and the changes are minimal, but they come at a cost

if we can I would prefer to have something more future proof and maintainable.

[ChristophWurst]

please squash? :)