[FormBuilderTextField] Add allowClear

[fehernyul]

Exactly the same like in FormBuilderDropdown.
I tried to make this, but the looking what it displays so wierd.

    child: Row(
      children: [
        Flexible(
          child: FormBuilderTextField(
            name: controlId,

            decoration: InputDecoration(
              labelText: field.dataType,
            ),

            validator: FormBuilderValidators.compose([
              FormBuilderValidators.required(ctx),
            ]),
            initialValue: field.initialValue,

            keyboardType: TextInputType.text,
          ),
        ),
        //const VerticalDivider(),
        InkWell(
          onTap: () {
            _formKey.currentState!.fields[field.fieldName]!.didChange('');
            //_formKey.currentState!.fields[field.fieldName]!.didChange(null);
          },
          child: Icon(Icons.clear),
        ),
      ],

[avdept]

Bumping on this. Would use it too

UPD: Turned out its pretty easy to add via decoration

decoration: InputDecoration(
          suffixIcon: IconButton(
                  icon: const Icon(Icons.clear),
                  onPressed: () => {},
                ))

[deandreamatias]

The answer of @avdept is correct

Currently we avoid created more parameters related with InputDecoration.
Instead, we will remove existent parameters on other fields like can see in this PR: #1078