Custom object items can be linked together on the import

[volha-pivavarchyk]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	[ ]
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 #...

⚠️ This PR needs changes from PRs #298, #308, #316 so it can be tested after merging them.

Description:

This PR add a new special field "Link Custom Items IDs" to the import form and allows to link imported items to existing custom items.

Steps to test this PR:

1. Open this PR for testing locally (see docs on testing PRs here)