feat: allow adding additional optional outs in c and c++ rules

[sfirmery]

C and C++ rules have optional outs for coverage.

This pr allow to add more optional outs for specific usage. For example c/c++ plugin of sonarqube produce additional outputs that can be kept using the new optional_outs c/c++ rules.

[peterebden]

Seems legit, although I would rather not add more features to most of the built-in rules at this point. Future development is going to happen in https://github.com/please-build/cc-rules - would you mind raising the change there please?

[sfirmery]

Yes, I've created a pr on the other repository https://github.com/please-build/cc-rules/pull/17

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