Introduce custom constraints to extend the smithy validation rules

[Gravewalker666]

Issue #, if available:
#1089

Description of changes:

• Introduced CustomConstraint interface for extensible validation rules via ServiceLoader
• Added Validator.CustomConstraintProvider to discover and register custom constraints as schema extensions
• Integrated applyCustomConstraints() into Validator.ShapeValidator to apply custom rules after standard validation
• Added ValidationError.CustomValidationFailure record for custom constraint violations

Benchmark numbers:

  Benchmark                                      Baseline (ns/op)    Feature (ns/op)    Improvement
  StringValidationBench.stringComposite          53.19               51.27              +3.6%                            
  StringValidationBench.stringEnumOnly           11.14               11.18              -0.4%                          
  StringValidationBench.stringLengthOnly         10.28               10.92              -6.2%
  StringValidationBench.stringNoValidation        9.90                9.90               0.0%
  StringValidationBench.stringPatternOnly        51.39               52.44              -2.0%
  ValidatorBench.person                          15.74               15.76              -0.1%
  ValidatorBench.pojoWithValidatedCollections   150.44              146.93              +2.3%
  ValidatorBench.unvalidatedPojo                 15.05               12.91             +14.2%
  ValidatorBench.validatedPojo                   17.83               14.01             +21.4%

A record of benchmark runs I've done along the way is documented here (non-complete): https://pastebin.com/GTuJKkQN

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[mtdowling]

Looks great! I'll merge after the stale comment fix.