Add new config option :use_yaml_unsafe_load

[andrykonchin]

Adds two new configuration options: use_yaml_unsafe_load and yaml_permitted_classes. They control how YAML-serialized fields are deserialized (YAML.unsafe_load vs. YAML.safe_load).

These options mirror ActiveRecord's configuration in Rails:

• config.active_record.use_yaml_unsafe_load
• config.active_record.yaml_column_permitted_classes

Previously, safe loading was unconditionally used on Ruby >= 3.1 with a hardcoded list of permitted classes ([Symbol, Set, Date, Time, DateTime]). Deserialization is now fully configurable while preserving the existing defaults for complete backward compatibility.

Previous PR - #889, #757.

[github-actions]

Package	Line Rate	Health
dynamoid	93%	➖
Summary	93% (4151 / 4462)	➖

Minimum allowed line rate is 90%

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.03%. Comparing base (ebde2e2) to head (a0a5d92).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/dynamoid/undumping.rb	71.42%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1051      +/-   ##
==========================================
- Coverage   93.07%   93.03%   -0.05%     
==========================================
  Files          84       84              
  Lines        4449     4462      +13     
==========================================
+ Hits         4141     4151      +10     
- Misses        308      311       +3     

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