[v2] fix: skip non-struct fields in extractIntoPtr slice path to prevent unmarshal panic - #3730

Backport: #3726

In extractIntoPtr function (link), the slice path iterates over all fields of the target struct and attempts to unmarshal the full JSON body into each one. When the target struct contains a non-struct field, such as a *bool (as described below), the code attempts to unmarshal a JSON object into it, causing a type mismatch error.

type PortExt struct {
	ports.Port
	portsecurity.PortSecurityExt
	portsbinding.PortsBindingExt

	PropagateUplinkStatusPtr *bool `json:"propagate_uplink_status,omitempty"`
}

for i := 0; i < newType.NumField(); i++ {
    // newType.Field(3) = PropagateUplinkStatusPtr (*bool)
	s := newType.Field(i).Addr()

	// Unmarshal is used rather than NewDecoder to also work
	// around the above-mentioned bug
	err = json.Unmarshal(b, s) // json.Unmarshal(b, **bool)
	if err != nil {
		return err
	}
}

This PR proposes a fix to only decode embedded structs in the loop, adding a check that skips non-struct types. All other field types are handled correctly by this ExtractInto call and by the final json.Unmarshal call after the loop.