feat(ci): add code coverage report

[sergey-melnychuk]

Closes #684
Closes #719

[sergey-melnychuk]

Both tarpaulin and llvm-cov turn out to be to heavy to run in a shared GitHub Actions worker - it fails with exit code 143. At this time I don't think it makes a lot of sense to migrate to a dedicated worker just for the sake of test coverage, as it can run locally.

For local run I personally prefer tarpaulin as it provides better branch coverage and generates a single-file HTML report:

export STARKNET_MAINNET_URL: https://starknet-mainnet.g.alchemy.com/starknet/version/rpc/v0_7/${ALCHEMY_KEY}
export STARKNET_SEPOLIA_URL: https://starknet-sepolia.g.alchemy.com/starknet/version/rpc/v0_7/${ALCHEMY_KEY}
BEERUS_TEST_RUN=1 cargo tarpaulin --out html
## Result: tarpaulin-report.html

Closing all relevant issues and PRs for now.