Support deserialization with System.Text.Json (#1928)

[JeremySkinner]

Fixes #1928

• Adds public constructor to ValidationFailure
• Adds setter to ValidationResult.Errors
  • Setter creates a copy of the list and removes nulls for consistency with the existing public constructor
• Makes the ValidationResult.RuleSetsExecuted setter public
• Minor cleanup to a few doc comments.
• Adds tests (ported from fix: json deserialization issue of ValidationResult #1935 with some modifications)