[v4] Support @azure/identity for authentication

[joheredi]

Update the v4 code generator to add a constructor overload that takes a TokenCredential instead of ServiceClientCredentials. Internally, this can create the credentials using the code snippet shared in Jon's adapter. This requires the generated code to take dependency on @azure/core-auth

For more context see: Azure/azure-sdk-for-js#11359

[ramya-rao-a]

This is done as part of #846