[rush] Add option to suppress hardcoded npmjs.org registry call

[dmichon-msft]

Summary

Add a new option to rush.json "suppressRushIsPublicVersionCheck" that disables a hardcoded call to the public npmjs.org registry to validate that the executing version of Rush (according to package.json) matches one that has been published to the public registry.

This feature is to support enterprises that run Rush on machines that they are denying access to npmjs.org.

Details

Also defer loads the WebClient wrapper around node-fetch to reduce startup time.

Updated rush-init template to include the setting.

How it was tested

Walked through rush install under debugger and validated that the condition hit both branches depending on the setting in rush.json.

Impacted documentation

Docs for rush.json