core/types: add stable API to compute tx intrinsic gas

Package types should have a way of computing the intrinsic gas of a transaction.

API Idea:

```go
func (tx *Transaction) IntrinsicGas(rules *params.Rules) uint64
```

This might also be useful for the data floor gas added in EIP-7623:

```go
func (tx *Transaction) DataFloorGas(rules *params.Rules) uint64
```