Support flags for reporting to CodeCov

[pascalberger]

For repositories with multiple components / test projects it would be useful if the individual reports can be uploaded to CodeCov with flags

[AdmiringWorm]

I am not sure if it makes too much sense adding this to the recipe build itself, this seems like it would become a never-ending problem to support properly.

I think it would be better for those that need this to update their codecov.yml configuration file instead to include the flags property.