rpmmd/repository: repoconfig pointers

Convert some of the fields in the `RepoConfig` struct
to pointers. Since `RepoConfig` will be used to convert
custom repositories to an array of `osbuild.YumRepository`,
we need to ensure that fields that are not set explicitly
are not saved to the `/etc/yum.repos.d` repository files.

internal/blueprint/repository_customizations_test.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/osbuild/osbuild-composer/internal/common"
 	"github.com/stretchr/testify/assert"
 )