perf(Spanner): Eliminate extend overhead

[alkatrivedi]

Removing unnecessary overhead from the extend dependency where simple shallow merges are sufficient to reduce CPU ticks and improvement in the performance of node client

[gemini-code-assist]

Code Review

This pull request replaces the external extend dependency with native Object.assign and explicit property assignments across various source and test files in the Spanner client library. The reviewer feedback is highly constructive, suggesting the complete elimination of the remaining extend deep copies by using shallow copies with Object.assign where safe. Additionally, the reviewer points out several redundant type assertions (such as casting gaxOpts and reqOpts) that can be removed to simplify and clean up the conditional property assignments.