Change to improve working with unkeyed records

[vlovgr]

This pull request aims to make it easier to work with unkeyed records.

If all records are unkeyed, simply use Unit serializers and deserializers. These are available implicitly as Serializer.unit and Deserializer.unit, so you can use ProducerSettings[Unit, V] and ConsumerSettings[Unit, V].

If some records are keyed and some are not, use Serializer#option and Deserializer#option, also available implicitly via Serializer.option and Deserializer.option. These serializers representNone as null in serialized form. Similarly, you can use these via ProducerSettings[Option[K], V] and ConsumerSettings[Option[K], V].

For more complex types and cases, there is Serializer.asNull[A] and Serializer.empty[A] which serializes arbitrary values of a given type as null or the empty Array[Byte], respectively.

For parity, similar additions have been made to HeaderSerializer and HeaderDeserializer.

Detailed changes:

• Add Deserializer#option, and Deserializer.option and unit.
• Add HeaderDeserializer#option and HeaderDeserializer.option and unit.
• Add Serializer#option, and Serializer.option, asNull, empty and unit.
• Add HeaderSerializer#option, and Serializer.option, asNull, empty and unit.

Fixes #96.

[codecov]

Codecov Report

Merging #97 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #97      +/-   ##
=========================================
+ Coverage   98.87%   98.9%   +0.02%     
=========================================
  Files          38      38              
  Lines         978    1004      +26     
  Branches       51      62      +11     
=========================================
+ Hits          967     993      +26     
  Misses         11      11

Impacted Files	Coverage Δ
src/main/scala/fs2/kafka/Serializer.scala	100% <100%> (ø)	⬆️
src/main/scala/fs2/kafka/HeaderDeserializer.scala	100% <100%> (ø)	⬆️
src/main/scala/fs2/kafka/Deserializer.scala	100% <100%> (ø)	⬆️
src/main/scala/fs2/kafka/HeaderSerializer.scala	100% <100%> (ø)	⬆️

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