Fix get_confluent_token callback for Managed Kafka service

[MaksYermak]

In this PR I have fixed the get_confluent_token callback function.
As mentioned in the documentation each callback function should have config_str parameter as default. Without this parameter this callback function doesn't work correctly and doesn't generate a token.

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[VladaZakharova]

hi @eladkal !
Can we please check if this one can be a part of 15.0.0 Google provider release? this one fixes something that we included in 14.1.0
Or it will be already part of 15.0.0+?

[VladaZakharova]

@potiuk
Hi there :)
Can you please checking these changes? Will help a lot!