Rewrite the Knative code-gen to a Go-native tool

## Expected Behavior

To fix https://github.com/knative/infra/issues/134 we need to remove the shell wrapper, so the Knative projects could run the code-gen in `go run ...` native way.

## Actual Behavior

[Current code gen](https://github.com/knative/pkg/blob/fcccb4e1a0a4d8a680d34665eb941a65376da2d3/hack/generate-knative.sh) has a shell wrapper. Because of it Knative projects which use this code gen require the vendor directory.

Start with hack/generate-knative.sh and review how the current shell wrapper invokes code generation. The work is complete when the wrapper is replaced by a Go-native tool that Knative projects can run with go run without requiring a vendor directory.

Written by the indexing model from the issue text.