feat: Improve contact merge to use database query instead of loading potentially many many items

[driskell]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	[x]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

Currently the contact item merge loads a huge amount of objects in some cases to migrate them to the new parent. It's better and faster to process as a query, similar to how all other merge operations are handled. It can process large amounts of items extremely quickly.

Steps to test this PR:

1. Open this PR for testing locally (see docs on testing PRs here)
2. Create a contact with a unique email
3. Add many custom objects
4. Submit a new contact by API that is anonymous
5. Update that contact using the same email
6. Merge should work (might need run command line)

• I don't know if these steps will work - for me I was testing with our own merge function *

[acquia-stalebot-platauto]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Please remove the stale label to avoid it being closed. Thank you for your contributions. More info: https://github.com/acquia/devops-github-administration/blob/main/docs/operations_related_to_repositories.md#acquia-stale-bot

[acquia-stalebot-platauto]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Please remove the stale label to avoid it being closed. Thank you for your contributions. More info: https://github.com/acquia/devops-github-administration/blob/main/docs/operations_related_to_repositories.md#acquia-stale-bot