Ci improvements

The intent for this PR is to make the build process compatible with the current OpenWRT version and to improve the CI process overall.

• Add a (.pre) stage to build the builder image to build the project and push it to the GitLab container registry. This will only be run on a schedule and if the BUILD_BUILDER_IMAGE is set
• Update Dockerfile for the builder image to debian:bookworm-slim, update dependencies
• Update default_config_multi to include all of the configuration options
• Run the pipeline using the recently updated container image using a matrix strategy for the given targets
• Add ./scripts/librerouteros_build.sh to build the project. This is called by the GitLab CI. Note that calling this with the DEBUG environment var set calls the build script with make -j1 V=sc for debug purposes. This is useful when using the GitLab cli to get extra debug information: glab ci run --variables-env "DEBUG:true"

Note that this is still very much a work in progress. More MRs will come.