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 *