Improve custom field select options validation

What does this MR do and why?

Use our own uniqueness validation so that we also validate uniqueness across unpersisted items

References

Related to #536186 (closed)

How to set up and validate locally

1. Create a select-type custom field and add options with the same value
2. After saving, it should show a user-friendly error message instead of a 500

MR acceptance checklist

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