[V4] Make the support for TokenCredentials behind a flag

v4 generates client constructors which takes a credential as input if the --add-credentials parameter was used. This credential is an alternate between the old cred type and the @azure/identity’s TokenCredential. However the implementation for TokenCredential in @azure/ms-rest-js does not support AAD for all packages because the scope is hard coded for management-plane packages only. This means packages listed in https://github.com/Azure/azure-sdk-for-js/issues/16663 can not use AAD when generated with v4. For more context, see https://github.com/Azure/azure-sdk-for-js/pull/16870. So instead, we should support TokenCredential behind a flag that is off by default so that those packages do not get the buggy support for AAD.

[deyaaeldeen]

v4 is still being used by a few data-plane packages (batch comes to mind) but @ramya-rao-a could help giving more context on the current situation.

[JonathanCrd]

After spending some time with this task and having a couple of discussions regarding this task and the version 4 of the generator, I have concluded that this issue should be closed for the following reasons:

• V4 is now deprecated, we are not introducing new features to it.
  • The development with v4x has become particularly problematic since many of its dependencies are now deprecated, not available anymore, and sometimes incompatible with today’s dev tools.
• This functionality is present in the newer version of the generator, and most packages have already migrated to it.
• This is not a customer reported issue, and it is not a critical fix.

Again, this issue does not represent a problem in v6, so packages that still depends on this can move to the newer version and get the latest features from the code generator.