(ci): add code coverage for unit tests in CI
Description
Sight lacks tests. The first step is to know which tests to create. To generate code coverage report will help us find which parts of the library isn't tested, and as such which tests we should create first.
Proposal
Add a new CI rule that will compile Sight with the appropriate flags and generate coverage data, and convert those data to human-readable reports, such as text, HTML and XML. I propose gcovr as the tool to generate those reports.