Allow additional flags to be passed to cppcheck
These can be specified in the .gitlab-ci.yml file (or if launching a
pipeline manually, via CI/CD -> Pipelines -> Run pipeline) using the
CPPCHECK_EXTRA variable. It is anticipated that this will be most useful
for specifying additional --suppress arguments.
Because library components routinely feature functions that are not called
from elsewhere within the library itself, the unusedFunction warning is
very commonly encountered in these components. To reduce the number of times
this needs to be specified in .gitlab-ci.yml files, CPPCHECK_EXTRA
defaults to --suppress-unusedFunction if it is unset for a library.
Because libraries are often listed as type C in ModuleDB, we instead
inspect the component name to see if it ends with Lib in order to activate
this functionality.
To avoid merge conflicts, this MR is not based on master: you should merge !12 first.