Add `CustomPlanModifiers` and `CustomValidators` functions to all `PlanModifiers` and `Validators` types

[bendbennett]

These changes are primarily to support updates in Remove attribute and block schema templates.

This PR adds convenience functions for obtaining CustomPlanModifiers and CustomValidators from PlanModifiers, and Validators types.

Additionally, convenience functions for obtaining CustomDefault from list, map, number, object, and set Default types have also been added.

For example:

BoolPlanModifiers

func (v BoolPlanModifiers) CustomPlanModifiers() CustomPlanModifiers {
    var customPlanModifiers CustomPlanModifiers

    for _, planModifier := range v {
        customPlanModifier := planModifier.Custom

        if customPlanModifier == nil {
            continue
        }

        customPlanModifiers = append(customPlanModifiers, customPlanModifier)
    }

    return customPlanModifiers
}

BoolValidators

func (v BoolValidators) CustomValidators() CustomValidators {
    var customValidators CustomValidators

    for _, validator := range v {
        customValidator := validator.Custom

        if customValidator == nil {
            continue
        }

        customValidators = append(customValidators, customValidator)
    }

    return customValidators
}

ListDefault

func (d *ListDefault) CustomDefault() *CustomDefault {
    if d == nil {
        return nil
    }

    return d.Custom
}

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