fix(common): polling policy clones initial state

[dbolduc]

Fixes #7853

Use the initial state of the polling policy as the prototype for clones. Add documentation and unit tests for GenericPollingPolicy.

Fixes #7562

@devbww made the connection to the Spanner samples.cc flake.

We make the polling policy out of a retry policy with a 60 minute timeout at the start of the test. But then when we clone the polling policy for individual polling loops, we are just reusing the original retry policy. So the test goes on, but after 60 mins, the retry policy is exhausted, and any polling loop will be terminated.

CloneWithPointers is the test for this flake, which now passes, but would previously have failed.

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: b997d1b14116567344f658f24cc737208725fbb8

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #7858 (b997d1b) into main (19b7ec7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7858   +/-   ##
=======================================
  Coverage   95.12%   95.13%           
=======================================
  Files        1284     1285    +1     
  Lines      116200   116280   +80     
=======================================
+ Hits       110539   110619   +80     
  Misses       5661     5661           

Impacted Files	Coverage Δ
google/cloud/polling_policy.h	100.00% <100.00%> (ø)
google/cloud/polling_policy_test.cc	100.00% <100.00%> (ø)
...ud/spanner/integration_tests/client_stress_test.cc	85.52% <0.00%> (-0.66%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	90.10% <0.00%> (+0.35%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 19b7ec7...b997d1b. Read the comment docs.