* 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
```
As recommended by https://golang.org/pkg/cmd/go/internal/generate/:
To convey to humans and machine tools that code is generated,
generated source should have a line early in the file that
matches the following regular expression (in Go syntax):
^// Code generated .* DO NOT EDIT\.$
Charlie Yin