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.tumbleweed

@@ -13,6 +13,7 @@ RUN zypper --non-interactive in --no-recommends \
   ccache \
   gcc \
   git \
+  lcov \
   glib2-devel \
   glibc-locale \
   gtk2-devel \