fix(core): use new aggregate grouping behavior in RelProtoConverter

[nielspardon]

changes RelProtoConverter to output the new and the old aggregate grouping proto structures

relates to #299 and substrait-io/substrait#1002

This PR changes the RelProtoConverter to output the new and the old grouping behavior keeping the existing io.substrait.relation.Aggregate behavior the same.

[benbellick]

Thanks for doing this! Will review when I have a free moment, but I think this would qualify as a breaking change, so it will need the ! in the PR title to follow conventional commit (plus the BREAKING CHANGE part of the message body).

[benbellick]

I think we will want to have a discussion about this one at the community sync. The way I see it there are two ways forwards:

• faster: We do what this PR suggests and just switch to the new representation (what this PR is doing), or
• safer: We have a release where we emit both the old and new representation, and only drop the old one in a subsequent PR.

I'm not sure what the right approach is, but probably worth talking about before merging.

EDIT: Thinking about it more, as long as Datafusion can handle the newer version already, then I am fine with just doing the "faster" change, considering all of the other libraries can already handle it.

[nielspardon]

I added code that outputs the old data structures in parallel as discussed in the community sync until we fully remove the old field via substrait-io/substrait#1002

[benbellick]

I think it looks good, just the one comment about making the tests a bit clearer. Thanks!

[benbellick]

LGTM, just a couple small comments. Thanks!