Alternative Code Coverage API

[c-mita]

A discussion thread for https://docs.google.com/document/d/1HWGRrzE17rVuCpSniA1W3MeqKl-puX_sEzqUGyMxW5E - a proposal to implement a new API for rules to implement code coverage that aims to better solve the problem of cross-language support.

Under this proposal, test rules would not need to know anything about how code coverage is implemented by dependent library or binary rules and the "_collect_cc_coverage" and "_collect_coverage" attributes would be obsoleted.

The target for this proposal is predominantly C++, but coverage for Rust and other rules may also benefit.