Add CodeCoverage::RspecReport and CodeCoverage::TestMap

What does this MR do and why?

Adds two classes needed to parse different files:

1. CodeCoverage::RspecReport: parses the content of an RSpec report
2. CodeCoverage::TestMap: parses the content of a test map, which maps source files to the test files that test them

The classes are 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

1. Fetch the branch locally

2. Fetch an RSpec report:

   curl -o rspec-11380416038.json https://gitlab.com/gitlab-org/gitlab/-/jobs/11380416038/artifacts/raw/rspec/rspec-11380416038.json

3. Fetch a test map file:

   curl https://gitlab-org.gitlab.io/gitlab/crystalball/packed-mapping.json.gz | gzip -d > mapping.json

4. Parse the 2 files with the classes 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.