ci: add code coverage workflow

[noahjoeris]

Description

This PR adds CI code coverage workflow. I tried to be as close to the bdk flow as possible. Adapted minor things to this repo.

Closes #8

[noahjoeris]

I just saw the issue linked to an older bdk code_coverage.yml. I'll try to update. -> Done

[ValuedMammal]

Thank you @noahjoeris just noting that I think there are plans to switch to codecov-action for uploading coverage reports using BDK's CODECOV_TOKEN (bitcoindevkit/bdk#2144), maybe we can try that here?

[noahjoeris]

Changed it to codecov. You might still need to set the codecov token tho :)

[ValuedMammal]

The token exists at the org level but maybe not activated for this repo yet. While I look into it here's some additional notes

• We can probably exclude unit/integration test modules from coverage as well
• I'm not sure what --doctests is doing and seems to have produced a warning but I didn't investigate
• It would be ideal to try cargo llvm-cov with a more recent version of the nightly compiler
• Unrelated, but we should also using caching in rust.yml which is made easy by actions-rust-lang/setup-rust-toolchain

[noahjoeris]

Good points thx. I updated.