New `coverage_report` keyword

Problem to solve

cobertura.xml is a legacy format for testing reports. As users are leveraging other types of coverage reports we want to be able to support and extend different kinds of coverage report formats easily. This will improve the discoverability of coverage reports.

Proposal

We'll first create a new artifacts:reports keyword coverage_report

# B. using coverage_report, explicit format
artifacts:
  reports:
    coverage_report:
     ​coverage_format: cobertura
     path: ​coverage/cobertura-coverage.xml