Missing old class ydb.iam.auth.IamTokenCredentials

[rekby]

ydb-python-sdk at version 2.1.3 has class ydb.iam.auth.IamTokenCredentials

Now (version 2.10.0) is missed.

It is problem while update old package.

Suggested fix:

1. Declare old IamTokenCredentials as alias of TokenServiceCredentials
2. Mark it as deprecated

[rekby]