ci: Introduce dependabot to manage dependency upgrades

[maschad]

The primary benefit is to be up to date on potential vulnerabilities, secondly I think it can potentially help reduce the friction involved doing fuel-core and forc upgrades, granted I don't know much of what's involved in that process, but at the very least having all our dependencies up to date can rule out those possibilities.

More info: https://docs.github.com/en/code-security/dependabot/working-with-dependabot