fix: Make BufferingPullSubscriber use List instead of ImmutableList

[dpcollins-google]

This makes it friendly to use in beam.

[codecov]

Codecov Report

Merging #256 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #256   +/-   ##
=========================================
  Coverage     72.43%   72.43%           
- Complexity      771      772    +1     
=========================================
  Files           140      140           
  Lines          4096     4096           
  Branches        215      214    -1     
=========================================
  Hits           2967     2967           
  Misses         1004     1004           
  Partials        125      125           

Impacted Files	Coverage Δ	Complexity Δ
...d/pubsublite/internal/BufferingPullSubscriber.java	82.05% <100.00%> (ø)	4.00 <0.00> (ø)

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