[2.3.0] Checkmk Werk 16640 created: Ruleset API: Improve custom validation

[//]: # (werk v2) # Ruleset API: Improve custom validation key | value ---------- | --- date | 2024-03-14T14:49:51+00:00 version | 2.3.0b4 class | fix edition | cre component | checks level | 1 compatible | no To better support the combination of different validation functions, FormSpecs now expect a sequence of validation functions instead of just one for `custom_validate`.