feat: add code coverage support

- Add --enable-coverage option to configure.ac
- Add coverage and coverage-html targets to Makefile.am
- Add coverage CI job with Codecov upload
- Add lcov to all Dockerfiles (arch, debian, fedora, tumbleweed, ubuntu)

Usage:
  ./configure --enable-coverage
  make check
  make coverage-html

Dockerfile.arch

@@ -17,6 +17,7 @@ RUN pacman -S --needed --noconfirm \
   check \
   cmake \
   cmocka \
+  lcov \
   curl \
   debuginfod \
   doxygen \