Use Goblint conf file

[sim642]

Currently all Goblint options are hard-coded:
https://github.com/goblint/MagpieBridge-Goblint/blob/5df41b6821e9b44d9f0f49dd6bd6c59e8b0eb456/src/main/java/GoblintAnalysis.java#L29

It would be better to use a Goblint .json conf file to allow tuning Goblint options without having to change the plugin itself.
The use of json-messages result should still be hard-coded instead of requiring everyone to duplicate that in their Goblint conf for anything to work.
Moreover, the plugin should not assume that the Goblint binary is in the current directory (./goblint) but rather just expect it to be in PATH (e.g. installed in the opam switch).

Therefore, a suitable command line could be something like:

goblint --conf goblint.json --set result json-messages -o output.json