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.