[Python] Add AwsSecret to the Secret utility

[shunping]

Descriptipon

Adds AwsSecret to retrieve secrets from AWS Secrets Manager via Secret.from_json(spec, 'AwsSecretsManager') or Secret.parse_secret_option('type:awssecret;name:...').

Changes

• AwsSecret: accepts secret_id/name/arn, version_id/version, version_stage/stage, plus any non-credential AwsOptions setting (e.g. region_name, endpoint_url) as a per-secret override.
• Secret.apply_pipeline_options(options): passed from from_json() / parse_secret_option() (and --gbek). AwsSecret uses it to pull credentials and defaults from AwsOptions lazily on workers.
• client_options: rejects AwsSecretsManager as --aws_credentials_secret_manager.

Part of #40528. Builds on #40547.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.95%. Comparing base (76f6e6d) to head (bc846b3).
⚠️ Report is 160 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40558       +/-   ##
=============================================
+ Coverage     56.16%   79.95%   +23.78%     
=============================================
  Files          1124      488      -636     
  Lines        178336    82416    -95920     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65893    -34277     
+ Misses        75645    16523    -59122     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.95% <100.00%> (+0.39%)	⬆️

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[github-actions]

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R: @damccorm for label python.
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