Subscriber syncCustomFieldValues handle serialized object and append values for arrays

[DevBayer]

I am creating this PR because I have identified a significant issue in the syncCustomFieldValues function in the following cases:

Case 1: When importing contacts with a "Multiple Select Choice" and attempting a bulk update, I found that the value in the column was being replaced instead of append.

With this PR, we address the following issue: wanting to delete all values of this type of Custom Field from a contact's profile via the Import tool. I believe that action (whether to append or replace) should be improved inside the form in the future for Custom Fields when performing such operations.

Case 2: When integrating with FluentForms and attempting to add a value to an existing array within a contact's record.