* go: replace objAPI-generated Pack func with method

See discussion at https://github.com/google/flatbuffers/issues/5668

* go: replace generated union type UnPack func with method

Similar to discussion https://github.com/google/flatbuffers/issues/5668

But signature:
```
func AnyUnPack(t Any, table flatbuffers.Table) *AnyT
```

Becomes,
```
func (rcv Any) UnPack(table flatbuffers.Table) *AnyT
```