ARROW-9520: [Rust] [DataFusion] Add support for aliased aggregate exprs

[returnString]

First attempt at contributing to Arrow/DataFusion, just fixing this small issue around aliased aggregate columns. Please let me know if this isn't an ideal implementation!

[github-actions]

https://issues.apache.org/jira/browse/ARROW-9520

[alamb]

Looks good to me. FYI @jorgecarleitao / @andygrove

[jorgecarleitao]

Thanks a lot for your first contribution to DataFusion!

This is great: really neat and clean solution to this, @returnString !

LGTM

[jorgecarleitao]

Can you run locally cargo fmt, so that we fix all linting issues shown in the CI pipeline?

[andygrove]

Thanks @returnString this looks great but you'll need to run cargo +stable fmt --all to fix the linter check in CI before we can merge.

[returnString]

That's interesting, I've got rustfmt set to run on save whilst editing in vscode but I was having some issues with code completion in the project too; I'll try and figure out what's up with that for any future contributions.

Are squash merges accepted for this project or would you prefer that I rebase the PR down to a single commit?

[alamb]

Are squash merges accepted for this project or would you prefer that I rebase the PR down to a single commit?

There is some sort of automation script that merges arrow PRs -- I don't think there is any need to rebase to a single commit

[andygrove]

LGTM. Thanks @returnString