build(ci): Basic Gradle workflow to build on Github

[skaldarnar]

Contains

A Github workflow to build, test, and analyze the code.

Uses Gradle Build Scans for reporting, see https://docs.gradle.com/enterprise/gradle-plugin/#connecting_to_scans_gradle_com (automatically accepted for CI runs, but no for local runs).

How to test

• ensure that gradlew distForLauncher --scan still prompts for user agreement
• ensure that the jobs look reasonable and work correctly (for instance, ❌ when build should be failing)

Additional Tasks

• configure mandatory checks to use Github jobs instead of Jenkins jobs for now