[coverage] Consider using `cargo-llvm-cov`

[ValuedMammal]

Describe the enhancement

Switch to using llvm-cov in .github/workflows/code_coverage.yml for generating coverage data. For example see bitcoindevkit/bdk#1986.

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Additional context

• grcov has some limitations including lacking support for branch coverage which makes it a less appealing option.
• Noise in coverage scores bdk#1984