CI: extract the code in .github/workflows/go.yml into a script

[ccoVeille]

The current code of .github/workflows/go.yml contains something like this

      - name: Detect and Test Submodules
        run: |
          # Find all directories containing a go.mod file within 'pkg'
          for module in $(find pkg -name "go.mod" -exec dirname {} \;); do
            echo "Testing module: $module"
            cd $module
          
            # Extract module name (replace '/' with '_')
            module_name=$(echo $module | tr '/' '_')

            # Download dependencies for the submodule
            go mod download
            go mod tidy

            # Run tests for the submodule and generate coverage
            export APP_ENV=test
            warning_count=$(go test ./... -v -short -coverprofile=${module_name}.cov -coverpkg=./... | tee /dev/tty | grep -c "WARNING" || true)
            total_warning_count=$((total_warning_count + warning_count))

            # Return to the root directory
            cd -
          done
          
          echo "Total warnings found: $total_warning_count"
          echo "total_warning_count=$total_warning_count" >> $GITHUB_ENV

The code is too complicated to be maintained in a YAML file, and it cannot be easily tested without running it via a GHA
The code could be extracted to something like a shell script or equivalent

This idea comes from the discussion that happened here