Add Rust format checker to CI

[TorstenStueber]

Currently the formatting of code in PRs is not checked and therefore we have a lot of improperly formatted code on main (e.g., the spacing around the / operator is missing here).

This leads to a situation where some PRs happen to be overly bloated as they happen to correct the formatting of some code files that might not even be related to the PR.

TODO

• add cargo fmt --check as a step to CI and let the action fail if it exits with non-0
• apply proper formatting to the complete repository

[TorstenStueber]

@pendulum-chain/product this is to keep the code base healthy and to make code reviews more straight forward.

[TorstenStueber]

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