fix: Two QOL issues with PubsubLiteIO

[dpcollins-google]

• UUIDs generated randomly are not valid UTF-8 strings so they cannot be read from the CPS shim by default
• Restrictions on bundle closing make it hard to test PSL with windowing pipelines

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Fixes #542 ☕️

[codecov]

Codecov Report

Merging #552 (bff816b) into master (39e04e4) will decrease coverage by 0.09%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master     #552      +/-   ##
============================================
- Coverage     72.70%   72.60%   -0.10%     
  Complexity      860      860              
============================================
  Files           156      156              
  Lines          4477     4483       +6     
  Branches        210      209       -1     
============================================
  Hits           3255     3255              
- Misses         1102     1107       +5     
- Partials        120      121       +1     

Impacted Files	Coverage Δ	Complexity Δ
...ogle/cloud/pubsublite/beam/SubscribeTransform.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...oogle/cloud/pubsublite/beam/SubscriberOptions.java	11.11% <20.00%> (+0.94%)	1.00 <0.00> (ø)
...in/java/com/google/cloud/pubsublite/beam/Uuid.java	84.61% <100.00%> (+2.79%)	3.00 <0.00> (ø)
...ubsublite/internal/BlockingPullSubscriberImpl.java	87.93% <0.00%> (-5.18%)	11.00% <0.00%> (-1.00%)

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