* Add broadcast API to Go forge scripts

Adds a hooks-based API to collect transactions broadcasted via `vm.broadcast(*)` in the Go-based Forge scripts. Users pass an `OnBroadcast` hook to the host, which will be called with a `Broadcast` struct with the following fields whenever a transaction needs to be emitted:

```go
type Broadcast struct {
	From     common.Address
	To       common.Address
	Calldata []byte
	Value    *big.Int
}
```

This API lets us layer on custom transaction management in the future which will be helpful for `op-deployer`.

As part of this PR, I changed the internal `callStack` data structure to contain pointers to `CallFrame`s rather than passing by value. I discovered a bug where the pranked sender was not being cleared in subsequent calls due to an ineffectual assignment error. I took a look at the implementation and there are many places where assignments to call frames within the stack happen after converting the value to a reference, so converting the stack to store pointers in the first place both simplified the code and eliminated a class of errors in the future. I updated the public API methods to return copies of the internal structs to prevent accidental mutation.

* Code review updates

* moar review updates

* fix bug with staticcall