SearchKit - Exclude serialized fields from implicit join selection

[colemanw]

Overview

SearchKit automatically makes certain fields available, e.g. email contact -> display_name. This type of join is not possible with serialized values (e.g. a multi-valued ContactReference custom field).

See https://lab.civicrm.org/dev/core/-/issues/2553#note_58551

Before

"Display Name" shown as an option for multi-valued ContactReference custom fields, but didn't actually work.

After

Still doesn't work, but not shown as an option.

[civibot]

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[seamuslee001]

This looks good to me

[seamuslee001]

Merging so that this gets into the RC, the code looks right to me