Generate these assets in config because this package is imported by
most.
Add internal/addgenheader as a simple helper because go-bindata doesn't
output the write header.
Also update test CI workflow to regenerate and check commit is clean.
Earlier versions transitively depend on a version of
github.com/matttproud/golang_protobuf_extensions which depends on the
now non-existent github.com/golang/protobuf/proto/testdata (breaking
change).
Move away from using dep to use Go modules which is production ready as
of 1.14.
Also tidy up README to make instructions for local development clearer
Fixes#385
