Support global Groovy variables in native mode

With the addition of [the support of global Groovy variables in Camel](https://issues.apache.org/jira/browse/CAMEL-19212) some changes are needed to support it in native mode as well.

Start by reading the linked CAMEL-19212 issue to understand the global Groovy variable support being added in Camel, then inspect how Camel Quarkus handles Groovy in native mode. Done means global Groovy variables work when the application is compiled and run in native mode, with the relevant native-mode coverage passing.

Written by the indexing model from the issue text.