Add 'gofumpt' check

[tstromberg]

Produce a list of files that should be "gofumpt'd", as well as a command to fix the files.

This will produce better output than golangci-lint currently does.

Consider filtering out .pb.go autogen files.

[mmlb]

I'd say we should do gofumpt on autogenerated files too. We can do it either as a second //go:generate directive or in the makefile recipe (latter is already being used in most cases I think).