Use ByteString instead of String for stats callback

[phile314]

This avoids the round-trip via String if one wants to pass the statistics JSON to aeson for further parsing.

Note that aeson has no decoding function accepting String nor Text, but only accepts ByteStrings.

See also #158

[sir4ur0n]

I don't use this feature so I don't have any strong opinion, though I guess it make sense not to force String on users.

Maybe alternatively we could provide the data type of statistics to help users, but this would definitely be another PR 😄

[phile314]

I have applied your comments, thanks.

Regarding the statistics type, I think without the FromJSON instances they are not terribly useful. And if we want FromJSON instances, hw-kafka-client will incur a dependency on aeson. Not that I have a problem with that, but it will increase the dependency footprint of hw-kafka-client. Though granted, for production systems one is likely to depend on Aeson for other reasons anyway.

[sir4ur0n]

@AlexeyRaga I don't know if you also saw this PR, I'm tagging you just in case

[AlexeyRaga]

LGTM, perhaps would require bumping a major version since switching to ByteString is a breaking change

[phile314]

I agree, bumping the major version is probably the right thing to do.