Add CodeCoverage::LcovFile

What does this MR do and why?

Adds a class that offers functionality to parse the content of a LCOV file.

The class is needed in the context of this work: !355 (merged).

Related to: https://gitlab.com/gitlab-org/quality/feature-readiness/team/-/issues/7

How to set up and validate locally

Probably not needed, but:

1. Fetch the branch locally

2. Fetch an LCOV file:

   curl -o gitlab.lcov https://gitlab.com/gitlab-org/gitlab/-/jobs/11380416469/artifacts/raw/coverage/lcov/gitlab.lcov

3. Parse the LCOV file with the class added in this MR

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.