WIP: PoC: bulk insert

What does this MR do?

Based on !22781 (merged). Closes #196166 (closed)

This "HACKS" to add a PoC of bulk insert:

1. This runs as part of regular .save!,
2. This gathers all objects for the given relation association and tries to save them in bulk (up-to 100),
3. This follows all callbacks and validations,
4. This is inefficient, as it runs very deep into stack trace, as we get another object to bulk insert (100) and try to save it,
5. It is very HACKY,
6. It does not work across different associations (relations assigned to different objects), as it only allows to bulk insert single relation.
7. Maybe we could do something simpler: mark some relations as BulkInsert, disallow usage of Callbacks on them, and simply overwrite the behaviour how they are persisted: doing it less hacky than proposed? (Or maybe still use callbacks, and all, but ignore after save/create)??

This runs in-line while doing .save! and gathers all object

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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