Implement basic CI flow

[sermazz]

This PR implements a basic CI flow divided (for now) in 2 workflows (.github/workflows):

• a workflow for public actions, taking care of linting for license headers, SystemVerilog codestyle, and C codestyle
• a workflow triggering runners on our internal GitLab, for regression tests using proprietary tools

The jobs executed by our internal runners are described in .gitlab-ci.yml.
For now, the hardware is built using QuestaSim and a basic test is executed.

CI failure is currently expected both for linting and regression test.
Next steps:

• extend with more advanced test strategy
• fix license headers and codestyle
• buildroot module, cloned by the cva6-sdk (in turn, used by the Cheshire dependency), sometimes cannot be cloned and causes the CI to randomly fail; I believe we can remove the dependency to the cva6-sdk from our custom branch of Cheshire

[Scheremo]

No comments from my end, LGTM!

[micprog]

Looks ok to me, some minor comments. Overall I think it would be important to ensure the CI is passing, though. If the baseline is that the CI is failing, then no updates will have to ensure the CI passes, rendering it useless and an easy way for bugs to get integrated.