Implement custom validation

[Skaiir]

Is your feature request related to a problem? Please describe.

Currently, users are force to define validations that are very constrained by our implementation. This is both wasteful for us in terms of implementation time, and restricting for users.

Describe the solution you'd like

We already extensively implement feel in the form playground, and we should keep heading this direction. A solution which would work nicely would be to allow for a feel-defined condition, as well as a feelers defined message which are evaluated as part of the validation cycle.

Describe alternatives you've considered

n/a