To make the configuration more intuitive and avoid exposing more
parameters to the user, we should:
1) Remove reset_by_subscriber as an enum constructor for
`offset_reset_policy`, as that might make the consumer hang
indefinitely without manual action.
2) Set the `begin_offset` `brod_consumer` parameter to `earliest` or
`latest` depending on the value of `offset_reset_policy`, as that’s
probably the user’s intention.
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