impl(bigquery): add attach_job method to query client

[haphungw]

Add BigQuery::attach_job to bind an existing out-of-process query job reference (JobReference) to a high-level Query handle without re-submitting SQL.

[gemini-code-assist]

Code Review

This pull request introduces the ability to configure a default Google Cloud project ID on the BigQuery client and adds an attach_job method to bind existing out-of-process query jobs. The feedback recommends refactoring the nested if-else block in attach_job using Rust's let-else syntax to reduce indentation, and replacing .unwrap() calls with .expect() in the new unit tests to adhere to the repository's style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.26%. Comparing base (e71018a) to head (7f66bb6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6295   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files         283      283           
  Lines       73009    73083   +74     
=======================================
+ Hits        70279    70353   +74     
  Misses       2730     2730           

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