chore: add default assignee for issues

[zoercai]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/blunderbuss.yml

[codecov]

Codecov Report

Merging #1065 (0112998) into master (1d4eed4) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1065      +/-   ##
============================================
- Coverage     85.24%   85.18%   -0.06%     
- Complexity     2658     2719      +61     
============================================
  Files           155      155              
  Lines         14453    14361      -92     
  Branches       1362     1361       -1     
============================================
- Hits          12320    12234      -86     
+ Misses         1565     1559       -6     
  Partials        568      568              

Impacted Files	Coverage Δ	Complexity Δ
...cloud/spanner/connection/ReadWriteTransaction.java	81.35% <0.00%> (-0.76%)	71.00% <0.00%> (+12.00%)	⬇️
...ud/spanner/SessionPoolAsyncTransactionManager.java	85.03% <0.00%> (-0.68%)	13.00% <0.00%> (+1.00%)	⬇️
...om/google/cloud/spanner/TransactionRunnerImpl.java	86.07% <0.00%> (-0.45%)	17.00% <0.00%> (+7.00%)	⬇️
...cloud/spanner/connection/SingleUseTransaction.java	90.90% <0.00%> (-0.42%)	46.00% <0.00%> (+10.00%)	⬇️
...java/com/google/cloud/spanner/AsyncRunnerImpl.java	93.54% <0.00%> (-0.40%)	11.00% <0.00%> (+2.00%)	⬇️
.../main/java/com/google/cloud/spanner/Operation.java	73.61% <0.00%> (-0.37%)	20.00% <0.00%> (+1.00%)	⬇️
...m/google/cloud/spanner/spi/v1/GapicSpannerRpc.java	80.22% <0.00%> (-0.34%)	89.00% <0.00%> (+17.00%)	⬇️
...panner/connection/AbstractMultiUseTransaction.java	94.11% <0.00%> (-0.33%)	8.00% <0.00%> (+1.00%)	⬇️
.../com/google/cloud/spanner/connection/DdlBatch.java	89.88% <0.00%> (-0.23%)	31.00% <0.00%> (+2.00%)	⬇️
...ain/java/com/google/cloud/spanner/SessionImpl.java	87.05% <0.00%> (-0.16%)	36.00% <0.00%> (+2.00%)	⬇️
... and 6 more

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[thiagotnunes]

@zoercai it seems this yml file is templated, meaning it will be overwritten on synth auto generated PRs. Could you check with the yoshi-java team how to make this change?