buildifier feature request: sort attribute dicts by mandatory=True, then by name

To improve usability for people reading the source code, we usually organize attrs such that all mandatory attributes are first, then the optional ones, e.g.
```
attrs = {
"required1": attr.whatever(mandatory=True),
"required2": attr.whatever(mandatory=True),

"optional1": ...
"optional2": ...
},
```

Is there a way to configure buildifier to honor that, short of just having it ignore the block? If I did the work, would a change like that be accepted?

Start by reading buildifier’s existing attribute-dictionary sorting behavior and its configuration options; the issue names no files or tests. Confirm the intended ordering of mandatory=True attributes before optional ones, with names sorted within each group, and verify the formatter’s output on representative attrs dictionaries.

Written by the indexing model from the issue text.