[GRPC] Bulk optimization and fixes

[karenyrx]

Description

A series of gRPC bulk API fixes/optimizations:

1. Fix update operation to use doc field instead of object field (still fallsback to object for backward compatibility)
2. Fix the default value of fetchSource to null to match REST
3. Add pipeline support for upsert
4. Optimize document bytes copying to pass bytes reference

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for f3818ca: FAILURE

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❕ Gradle check result for 34bbaf4: UNSTABLE

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[codecov]

Codecov Report

❌ Patch coverage is 91.83673% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.24%. Comparing base (2f930dc) to head (2fa7426).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...est/document/bulk/BulkRequestParserProtoUtils.java	91.66%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #19937   +/-   ##
=========================================
  Coverage     73.24%   73.24%           
- Complexity    71519    71561   +42     
=========================================
  Files          5789     5789           
  Lines        327121   327142   +21     
  Branches      47117    47120    +3     
=========================================
+ Hits         239591   239608   +17     
- Misses        68269    68324   +55     
+ Partials      19261    19210   -51     

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[github-actions]

❌ Gradle check result for 6d897b8: FAILURE

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❌ Gradle check result for 6d897b8: null

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[github-actions]

❌ Gradle check result for 2a16f30: FAILURE

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❕ Gradle check result for 2a16f30: UNSTABLE

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[github-actions]

✅ Gradle check result for 3b80f63: SUCCESS

[msfroh]

Looks good to me overall. I really appreciate the use of zero-copy for the byte arrays.

I'll hold off on merging until you've had a chance to respond to @varunbharadwaj's comments.

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❌ Gradle check result for 9e0aae9: FAILURE

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❌ Gradle check result for 2fa7426: FAILURE

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✅ Gradle check result for 2fa7426: SUCCESS