feat: add ModelRepository#_UNSTABLE_bulkDestroy and manual ON DELETE handling

[ephys]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
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Description Of Change

This PR introduces ModelRepository#_UNSTABLE_bulkDestroy, as part of the migration to repositories & general rewrite of Model

This PR also introduces the manualOnDelete option in both _UNSTABLE_bulkDestroy and _UNSTABLE_destroy.

This PR only implements that part of the code that relies on DELETE queries, as the rest will require implementing _UNSTABLE_update first, which depends on #17063

Closes #2586

[WikiRik]

There are still a few TODOs and commented out code here. Is that all going to be fixed when _UNSTABLE_update is implemented? Maybe we should wait for that and split the new manualOnDelete option for _UNSTABLE_destroy to another PR

[ephys]

_UNSTABLE_update cannot be done until #17063 is merged, so this PR would be blocked for a while even though the base feature is ready. I'm also trying to do smaller PRs to slowly build the feature instead of providing it in one massive PR, and implementing delete methods was a much smaller task than implementing update first, which lets me introduce the base concepts before introducing the huge method that update is going to be (as, itself, requires things like a new validator, another big task)

[WikiRik]

Plenty of TODOs, but it's better to merge this now instead of having it blocked for a while.