style: fix and enforce formatting

[danieleades]

applies the (already configured) rust formatting, as specified in the CONTRIBUTING.md guide

also adds a github workflow to check the formatting on PRs, and a dependabot config to keep the CI job up-to-date

[mmstick]

No need. Any formatting should be done in an active PR to avoid unnecessary system updates and loss of QA time.

[danieleades]

No need. Any formatting should be done in an active PR to avoid unnecessary system updates and loss of QA time.

But... It isn't getting done in active PRs, as evidenced by the formatting needing to be applied.

I would also question a QA process which is burning time on format-only changes. That sounds pointless.

And finally, the primary purpose of this PR is to introduce a CI check for formatting- which will neither be included alongside feature PRs or impact QA

[mmstick]

It is, but I only format files that I have modified.

[jacobgkau]

I think the idea with a CI check is that it would be a one-time thing to fix current formatting issues, and after that, it would just be ensuring it is actually being done in the active PRs. (In one of my recent PRs, I noticed rustfmt was suggesting a change, but I kept it as-is to better match code that was already present; this check would've forced me to run rustfmt and accept the change, which our contributing guide says I should've done.)

Now, this is still extremely low-priority and can wait until after #380, since it has no impact on the actual functionality of the code. But I don't see why it's a bad idea, if we intend to comply with the official formatter's formatting. (If we had our own style we wanted to follow instead of rustfmt's style, that would be different.)