Add support for multi-value contact reference custom fields

[colemanw]

Overview

Allows custom fields of type ContactReference to be multi-valued. See also https://lab.civicrm.org/dev/core/-/issues/2123

Before

Contact reference fields must be single-valued.

After

Multi-valued contact reference custom fields supported.

[civibot]

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[michaelmcandrew]

@colemanw - I have tested this in a few scenarios and it is working nicely for the client. I think it is ready to merge.

[michaelmcandrew]

• General standards
  • (r-explain) Pass
  • (r-user) Pass: This is opt in. No change for existing users.
  • (r-doc) Pass
  • (r-run) Pass: I have tested this for a week on a client site, including data entry and advanced search. Came across some issues early on which were then fixed and tested in a subsequent round of testing.
• Developer standards
  • (r-tech) Pass: Written by Coleman extending usage of a pre-existing pattern, so likely safe.
  • (r-code) Pass
  • (r-maint) Pass
  • (r-test) Pass: Includes some new tests.