Add LCOV coverage reporting as a replacement for COVERALLS reporting

[jgebal]

Existing COVERALLS reporting is no longer easy to publish to COVERALLS.
If users would be using GithubActions/AzurePipelines etc., the preferred format would be LCOV.

The preferred way to publish coverage reports to Coveralls is via Coveralls App and this one accepts LCOV format.

The problem was discovered while trying to switch from Travis to GithubActions.
In fact, even existing Travis integration was not trivial as it required additional scripting on CI side to make the output JSON report consumable by Coveralss by adding MD5 hash for each source file reported.

A discussion on Coveralls led me to conclusion that the best approach would be to implement a new LCOV coverage reporter.

[jgebal]

Description of LCOV format can be found here
A good interpretation of it can be also found in this parser, particularly in this file

[lwasylow]

Don't we already publish code coverage in sonar cloud? Just wonder if the lcov is a niche? I think most people using Jenkins or azure which consume cobertura. Just wonder how much uptake it will have and how many people actually using this format?