fix: Change Kafka{Producer/Consumer} to fix the number of partitions

[dpcollins-google]

…instead of looking it up.

This prevents making too many queries to the number of partitions.

[codecov]

Codecov Report

Merging #322 into master will increase coverage by 0.04%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master     #322      +/-   ##
============================================
+ Coverage     71.36%   71.41%   +0.04%     
+ Complexity      875      874       -1     
============================================
  Files           153      153              
  Lines          4722     4712      -10     
  Branches        227      228       +1     
============================================
- Hits           3370     3365       -5     
+ Misses         1225     1220       -5     
  Partials        127      127              

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsublite/kafka/ConsumerSettings.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...oogle/cloud/pubsublite/kafka/ProducerSettings.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...gle/cloud/pubsublite/kafka/PubsubLiteProducer.java	79.36% <66.66%> (+1.42%)	16.00 <1.00> (ø)
...gle/cloud/pubsublite/kafka/PubsubLiteConsumer.java	72.14% <75.00%> (+1.29%)	53.00 <1.00> (ø)
.../google/cloud/pubsublite/kafka/SharedBehavior.java	77.77% <100.00%> (-4.05%)	3.00 <0.00> (ø)

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