Proposal: introduce cleaner APIs for starting workers

[k32]

Currently brod API functions for starting worker processes pass each individual option as a function argument. However, number of arguments tends to get large as more options are added, which leads to the code that is fairly hard to read. I suggest to pass all options in a single map instead, in the same way it's done here: https://github.com/klarna/brod/blob/master/src/brod_group_subscriber_v2.erl#L168

Old APIs will be kept for backward compatibility.