Setup dependabot for opening PRs to update gomod dependencies

[neelvirdy]

This automates opening of PRs to update gomod dependencies. Made myself the default reviewer, happy to add anyone else who'd like to look at these. Ideally, we'd eventually allow auto-merging of these PRs as long as they still pass checks.

Inspired by station's setup: https://github.com/filecoin-station/filecoin-station/blame/main/.github/dependabot.yml

Other resources:

• Core docs: https://github.com/dependabot/dependabot-core
• Gomod example: https://gist.github.com/magnetikonline/6f215db058e327905bce66c37f92426c

This also needs someone with repo admin perms to enable dependabot PRs. @jlogelin @alvin-reyes are one of you able to do this? Looks very quick to do. Docs: https://docs.github.com/en/code-security/dependabot/dependabot-security-updates/configuring-dependabot-security-updates#enabling-or-disabling-dependabot-security-updates-for-an-individual-repository