GitLab

This MR enables offline builds.

• Adds new CI job create cpm cache which uploads a cache to the package registry. Runs only on master and when the cache needs to be updated.
• Adds new CI job offline configure which tests using the cache. Runs only on master. See this test run.
• New docs page at https://www.opengeosys.org/docs/devguide/advanced/offline-build (not live yet).

TODO in later MR (because based on ExternalProject_Add() and not cpm):

• MFront
• PETSc

Fixes #3200.

1. Feature description was added to the changelog
2. Tests covering your feature were added?