feat: add code coverage with branch coverage

- Add --enable-coverage option to configure.ac
- Add coverage/coverage-html targets to Makefile.am
- Add coverage CI job with lcov + Codecov upload
- Include both unit and functional tests in coverage
- Enable branch coverage (step towards MC/DC)
- Add lcov to all Dockerfiles
- Add coverage/ to .gitignore

Dockerfile.debian

@@ -12,6 +12,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   ccache \
   gcc \
   git \
+  lcov \
   libcmocka-dev \
   libcurl3-dev \
   libgcrypt-dev \