Add target summary

[darkrift]

Something very useful when a build fails in CI is to be able to see the target informations/coverage and logs, especially when test fails.

Information that could be added :

• Number of test run
• stats and logs per run
• rule type
• coverage information
• target outputs
• artifacts with their download links (test.log, test.xml, test.lcov, etc)

Buildbuddy has a very nice target reporting that is super useful when test fails. It also provides coverage information, target/test logs, links to artifacts and target outputs, etc.

While we can use --test_output errors, it really bloats console output.

[meroton-benjamin]

Target level data is supported as of the latest version. This issue sounds like visualizations of eventual issues that could be very useful, but as a single ticket I think it might be too broad. Would you mind breaking down this into multiple smaller more actionable tickets?