feat: upgrade to go 1.18 and use workspaces to manage the handler

[LucasRoesler]

Replace the custom bash script with a Go workspace, to handle setting the correct imports and replacements during the Go builds. This is simpler and uses official Go tooling.

How Has This Been Tested?

I have started a repo of test functions here: https://github.com/LucasRoesler/golang-http-template-examples

The first example publisher is an example go-http function that publishes messages to a NATS subject. This function demonstrates using

• external public packages: github.com/nats-io/nats.go and github.com/openfaas/templates-sdk/go-http
• it contains a named subpackage pkg/version

How are existing users impacted? What migration steps/scripts do we need?

Checklist:

I have:

• [ ] updated the documentation and/or roadmap (if required)
• [x] read the CONTRIBUTION guide
• [x] signed-off my commits with git commit -s
• [ ] added unit tests