Allow specifying a particular NPM registry for TfxInstaller task to use

[Porges]

See comments over here for more background: #476 (comment)

It would be useful if TfxInstaller provided a registry option to select which NPM registry to install from. At the moment TfxInstaller is unusable in some configurations where (for example) npmjs.org is blocked.

(Auth should also automatically happen for this registry if it's in the same ADO project, just like in the Npm@x task.)

[jessehouwing]

This relates to: https://github.com/microsoft/azure-devops-extension-tasks, not tfs-cli.

[jessehouwing]

You can always manually populate the Tools cache folder, the TfxInstaller task will pick this up.

[Porges]

This relates to: https://github.com/microsoft/azure-devops-extension-tasks, not tfs-cli.

Will migrate it to there.