rustfmt.toml for SPL

[buffalojoec]

This PR adds a rustfmt.toml to the SPL repository.

I've included only the file itself with configs for review. Once we agree on the configs for the whole workspace, I can run the formatter and PR the changes.

[buffalojoec]

Note this will also include the mandatory use of nightly, thus CI modifications will be required as part of this change.

[joncinque]

How about we embrace these different features one by one? It seems like this will be chaos if we just enable all of them in one commit.

Let's start with group_imports and imports_granularity, and then we can see how each option impacts the repo before adding it.

Maybe we can even start with using nightly format in CI before adding this file. What do you think?

[buffalojoec]

Let's start with group_imports and imports_granularity, and then we can see how each option impacts the repo before adding it.

Maybe we can even start with using nightly format in CI before adding this file. What do you think?

yeah this is going to result in a bazillion changes so I'm in favor of splitting it up.

[buffalojoec]

Closing in favor of #5741