mvc: fields should implement getCurrentValue() rather than __toString()

[fichtner]

To ease this effort avoid having derived classes implement both __toString and getCurrentValue for consistency across value return. Also only ever return strings (empty if not otherwise possible).

[fichtner]

Included in #8162

[AdSchellevis]

I think this is ok, the tricky/risky one is UpdateOnlyTextField if I'm not mistaken, but as long as the child class overwrites __String() we should be good.

core/src/opnsense/mvc/app/models/OPNsense/Base/FieldTypes/UpdateOnlyTextField.php

Lines 41 to 44 in 1e78885

	public function __toString()
	{
	return "";
	}

Although I do expect some impact in a couple of constraints (likely introduced here 8572171)

[fichtner]

Thanks, let this sit till January when I can test better.