proposal: log/slog: add iterator form of `Record.Attrs` 

`log/slog.Record.Attrs` is used to iterate over the attributes stored in a record:
```
func (Record) Attrs(f func(Attr) bool)
```

If #61897 is accepted, we propose adding a method that fits better with that proposal:
```
func (Record) AllAttrs() iter.Seq[Attr]
```