interop: improve rpc_soak and channel_soak test to cover concurrency in Go

[zbilun]

RELEASE NOTES:

• Improved the rpc_soak and channel_soak tests to support concurrency in Go. This change increases the test coverage of the C2P E2E load test and follows a similar approach used in the Java implementation.

This PR aims to enhance the test coverage of the C2P E2E load test by improving the rpc_soak and channel_soak tests to support concurrency in Go. The updated logic closely follows the approach used in the Java implementation, which has already been merged.

rpc_soak:
The client performs many large_unary RPCs in sequence over the same channel. The test can run in either a concurrent or non-concurrent mode, depending on the number of threads specified (soak_num_threads).

channel_soak:
Similar to rpc_soak, but this time each RPC is performed on a new channel. The channel is created just before each RPC and is destroyed just after.

Note on Concurrent Execution and Channel Creation:
In a concurrent execution setting (i.e., when soak_num_threads > 1), each thread performs a portion of the total soak_iterations and creates and destroys its own channel for each RPC iteration.

Thread-specific logs will include the thread_id, helping to track performance across threads, especially when each thread is managing its own channel lifecycle.

@townba
@dfawley

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.02%. Comparing base (66ba4b2) to head (823ae79).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7926      +/-   ##
==========================================
- Coverage   82.04%   82.02%   -0.03%     
==========================================
  Files         377      381       +4     
  Lines       38180    38539     +359     
==========================================
+ Hits        31326    31612     +286     
- Misses       5551     5609      +58     
- Partials     1303     1318      +15     

see 50 files with indirect coverage changes