Support account-level exclusions from named-blob cleanup

[zichengl]

Summary

Adds an account-level exclusion to the named-blob stale-data cleanup runner, complementing the container-level exclusion added in #3286.

A new config property frontend.named.blob.cleanup.excluded.accounts takes a comma-separated list of accountName entries. Every container under a listed account is skipped by the cleanup runner, so its superseded (stale) named-blob versions are retained rather than deleted — useful for exempting an entire account without enumerating each of its containers.

Design

• Separate property rather than overloading ...excluded.containers with slash-less entries. Keeping them distinct means a missing "/containerName" (a typo) can never silently widen a single-container exclusion into a whole-account one.
• The runner checks the account-level set first, then the container-level set. The existing fail-open behavior on an unresolvable account is unchanged (an account that can't be resolved never suppresses cleanup).
• Same parsing rules as the container list: per-entry whitespace is trimmed, blank entries dropped, and whitespace within a name stays significant.

Testing

• FrontendConfigTest: parsing/trimming of the new accounts list.
• NamedBlobsCleanupRunnerTest: an account-level exclusion skips every container under that account while a container in another account is still cleaned.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.81%. Comparing base (52ba813) to head (f35c0c3).
⚠️ Report is 411 commits behind head on master.

Files with missing lines	Patch %	Lines
...github/ambry/frontend/NamedBlobsCleanupRunner.java	30.76%	8 Missing and 1 partial ⚠️
...n/java/com/github/ambry/config/FrontendConfig.java	83.33%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3290       +/-   ##
=============================================
- Coverage     64.24%   50.81%   -13.43%     
+ Complexity    10398     8704     -1694     
=============================================
  Files           840      938       +98     
  Lines         71755    80505     +8750     
  Branches       8611     9691     +1080     
=============================================
- Hits          46099    40912     -5187     
- Misses        23004    36205    +13201     
- Partials       2652     3388      +736     

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