[Task] Add coverage workflow

## Description
We should start using the frequently recommended cargo-llvm-cov crate.

## Resources
[cargo llvm cov](https://github.com/taiki-e/cargo-llvm-cov)
[PR that removed the old coverage workflow](https://github.com/iotaledger/identity.rs/pull/1083)

## Change checklist
Add an `x` to the boxes that are relevant to your changes, and delete any items that are not.

- [ ] The feature or fix is implemented in Rust and across all bindings whereas possible.
- [ ] The feature or fix has sufficient testing coverage
- [ ] All tests and examples build and run locally as expected
- [ ] Every piece of code has been document according to the documentation guidelines.
- [ ] If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
- [ ] If the feature is not currently documented, a documentation task Issue has been opened to address this.