Registry tool: Use `registry config` instead of env vars for scripts

[theganyo]

Per https://github.com/apigee/registry/issues/625#issue-1282776780:

This would probably replace the scripts in the auth directory with direct registry subcommands. auth/HOSTED.sh might become registry config gcloud and auth/LOCAL.sh might be registry config local (with defaults and flags for pointing to a locally-running server).

Update client-side, customer-facing scripts and/or docs to use registry config instead of env vars.