Event Streams: support sarama client with IAM token

[JunliWang]

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related with #5799
when platform supports trusted profile as authentication method, apikey config is not present, then Event Streams' kafka client(sarama) will use token for authentication.
This PR configures sarama to use sasl.mechanism=OAUTHBEARER when token is available and valid.

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...