Enable Dependabot

[kou]

We only need this part: https://github.com/apache/arrow/blob/a6fe59592c643d591ee719041dd2b5d8b9ada7d6/.github/dependabot.yml#L20-L33

[raulcd]

@lidavidm @kou , I've merged this and a bunch of dependabot PRs were opened. We probably shouldn't merge any of those until we have a running CI