ci: enforce formatting, go mod tidy

[cwaldren-ld]

Adds CI checks for:

• gofmt: important to ensure code is always formatted in the repo
• go mod tidy: important to ensure deterministic dependencies

It does this via git/hooks/pre-commit, which can also be installed via make install-hooks.

[dbolson]

Would it make more sense to run these in a pre-commit hook and call the pre-commit hook in a GH action? Then we could get immediate, and skippable, feedback and use the same code for the CI build.

[cwaldren-ld]

Yea, that'd probably be better - I didn't do it because I'm not a fan of pre-commit. But I can definitely make that change.

[cwaldren-ld]

Ok, refactored to be a pre-commit hook.