`generate-config` enhancements

[Elson9]

Enhancements

• allow user to specify template via flag then provide additional parameters via interactive
  • user would then still need to set service and upstream
  • Check library for back-filling template flag for interactive mode.
  • Discussion of this feature ticket comes from Namespace to Gateway - (CLI) Support users creating services and routes without name conflicts #101
• provide https://httpbin.org as a default option for upstream (i.e. in the interactive, user can hit ENTER to select this default)

[rustyjux]

extended this issue with functionality around blank org blocking validation

[rustyjux]

split out bugs into #127 and #128 so they can potentially be prioritized over the enhancements in this issue.