Add input filter validation to fieldset tutorial

[Redcentaur]

In the in-depth tutorial you add the use of laminas-form and laminas-form-fieldset and in the Controller check for validity of the inputs. In the conclusion of that section, it is claimed that we've learnt how to validate input but we haven't: the input to the fields is not validated because we haven't created an input filter or attached any validators to the form or to the fieldset containing the form data.

I think it is particularly important to cover this topic as you are using fieldset in the form for the object hydrator and this can make setting data on the form and collecting data back from valid forms difficult with input filters. Can you cover it in more detail please?