Make it possible to configure max_send_batch_size in GCP pubsub.

[bianpengyuan]

Same as max_recv_batch_size, make it possible to customize max_send_batch_size at GCP pubsub package.

[google-cla]

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[bianpengyuan]

Hi @vangent could you please take a look? Thank you!

[bianpengyuan]

@vangent gentle ping, could you please take a look? Thank you!

[codecov]

Codecov Report

Merging #3257 (47fa19e) into master (28b1328) will increase coverage by 0.04%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master    #3257      +/-   ##
==========================================
+ Coverage   77.37%   77.41%   +0.04%     
==========================================
  Files         103      103              
  Lines       13544    13554      +10     
==========================================
+ Hits        10479    10493      +14     
+ Misses       2338     2335       -3     
+ Partials      727      726       -1     

Impacted Files	Coverage Δ
pubsub/gcppubsub/gcppubsub.go	80.68% <78.57%> (-0.03%)	⬇️

... and 3 files with indirect coverage changes