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)

[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. Thank you for your contributions.

[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

[PierreAmmeloot]

@escopecz how can I help you with this pull request?

[escopecz]

The conflict must be resolved and then it must go through our product team to allocate the resources quality checks. CC @gunnrryy

[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

[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