[IPC/Go] Flatbuffers package declared as internal - Useful with custom MessageReaders

### Describe the enhancement requested

Hi,

While building a custom MessageReader I realized that the flatbuffers package in go is declared internal. Yet, this is of importance when reading the header metadata and to determine the size of the body when parsing IPC messages.
I would be curious if there is any reason why you declare that internal, and if not could that be changed?

As far as I got it from the docs, creating custom IPC MessageReaders is something you consider a valid use case:

https://pkg.go.dev/github.com/apache/arrow/go/arrow@v0.0.0-20211112161151-bc219186db40/ipc#NewReaderFromMessageReader
```
NewReaderFromMessageReader allows constructing a new reader object with the provided MessageReader allowing injection of reading messages other than by simple streaming bytes such as Arrow Flight which receives a protobuf message
```

### Component(s)

Go