feat(storage): add support for partial list bucket

[bajajneha27]

This change introduces support for partial list buckets in both gRPC and JSON transports.

We've introduced a new optional boolean flag, returnPartialSuccess. By default, the flag is false.
If the flag is set to true, the API will return a list of buckets and will additionally populate a separate list of the unreachable bucket names.

[codecov]

Codecov Report

❌ Patch coverage is 97.68340% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.94%. Comparing base (4ac3fe1) to head (698f83d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...cloud/storage/list_buckets_extended_reader_test.cc	97.16%	3 Missing ⚠️
...e/cloud/storage/examples/storage_bucket_samples.cc	89.47%	2 Missing ⚠️
google/cloud/storage/client.h	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15763      +/-   ##
==========================================
- Coverage   92.94%   92.94%   -0.01%     
==========================================
  Files        2457     2458       +1     
  Lines      227084   227342     +258     
==========================================
+ Hits       211066   211299     +233     
- Misses      16018    16043      +25     

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[v-pratap]

Should we use std::move() here?

[bajajneha27]

You mean on kv.value() ?

[scotthart]

The nlohmann_json library may limit the ability to move things. I'm pretty sure value() returns a copy of what the kv iterator is pointing at. However, as result.unreachable is a std::vector<std::string>, emplace_back isn't helping here, push_back(T&&) would be preferable as it will move whatever std::string the json library creates.

[v-pratap]

Do we need to add tests for the case where ReturnPartialSuccess is false or empty?

[bajajneha27]

By default it's false and we have existing testcase covering that.
If you think we should do it and we're doing it across languages then I can add.

[scotthart]

The nlohmann_json library may limit the ability to move things. I'm pretty sure value() returns a copy of what the kv iterator is pointing at. However, as result.unreachable is a std::vector<std::string>, emplace_back isn't helping here, push_back(T&&) would be preferable as it will move whatever std::string the json library creates.