Add headers support

[tvoinarovskyi]

Fixes #462

[codecov]

Codecov Report

Merging #471 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
+ Coverage   98.35%   98.37%   +0.02%     
==========================================
  Files          31       31              
  Lines        4866     4870       +4     
==========================================
+ Hits         4786     4791       +5     
+ Misses         80       79       -1

Impacted Files	Coverage Δ
aiokafka/consumer/fetcher.py	96.96% <ø> (ø)	⬆️
aiokafka/structs.py	100% <ø> (ø)	⬆️
aiokafka/producer/producer.py	99.49% <100%> (+0.01%)	⬆️
aiokafka/producer/message_accumulator.py	98.88% <100%> (ø)	⬆️
aiokafka/record/legacy_records.py	98.48% <0%> (+0.37%)	⬆️

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