[CLIENT-3805] Add missing app_id attribute to Cluster class for extended metrics

[DomPeliniAerospike]

There is missing code coverage for returning None from the Cluster class's app_id attribute, but that is fine because it is hard to reproduce that behavior and it's documented that as_cluster.app_id can be NULL in the C client

Extra changes:

• Allow client config option app_id to be set to None.
• Docs: fix app_id and cluster_names' types to be Optional[str] since they can be set to None.
• Docs: set default value for client config's app_id and cluster_name option to None.
• Docs: Update outdated documentation for the behavior of client config's cluster_name.
• Make a standalone script for user agent test to make easier to run locally
• In user agent standalone test, also check the server received the app_id from the Python client's client config

Docs

https://aerospike-python-client--864.org.readthedocs.build/en/864/aerospike.html#:~:text=meaning%20‘never%20compress’-,cluster_name,-(Optional[str
https://aerospike-python-client--864.org.readthedocs.build/en/864/aerospike_helpers.metrics.html#aerospike_helpers.metrics.Cluster.app_id

TODO

• cluster_name description needs to be tested
• app_id documentation needs improvement.
• test case where app_id is not-set
• script needs to clean up properly when cancelled

Manual testing

• Code coverage looks good
• Build artifacts passes
• Valgrind shows no mem errors or leaks from these changes
• Massif mem usage looks ok

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.37%. Comparing base (a60d3d9) to head (92bda5a).
⚠️ Report is 10 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/main/conversions.c	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #864      +/-   ##
==========================================
- Coverage   83.38%   83.37%   -0.01%     
==========================================
  Files          99       99              
  Lines       14394    14406      +12     
==========================================
+ Hits        12002    12011       +9     
- Misses       2392     2395       +3     

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[justinlee-aerospike]

Changes are okay

Justin already reviewed and approved