BUG: Add FieldSet.constant_model attribute

[VeckoTheGecko]

Description

Fixed a bug that resulted in shared global state between fieldsets when working with constant fields.

The Models used for the constant fields now live on the FieldSets themselves.

Checklist

• Closes Remove global state constant field models #2736
• Tests added - **to some extent. I want to add more tests in Add testing for adding fields to FieldSets #2727 **
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

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