Add additional fields support to codegen/features

[muXxer]

This PR adds support for additional fields in the features codegen.

For example the command:

//go:generate go run github.com/iotaledger/hive.go/codegen/features/cmd@develop identifier.tmpl ../identifier_anchor.gen.go AnchorID a "chainid" "AddressType=AnchorAddress"

and in the template:

func ({{.Receiver}} {{.Name}}) ToAddress() ChainAddress {
	var addr {{.AddressType}}
	copy(addr[:], a[:])

	return &addr
}

would result in:

func (a AnchorID) ToAddress() ChainAddress {
	var addr AnchorAddress
	copy(addr[:], a[:])

	return &addr
}