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.
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