Address S3956: "Generic.List" instances should not be part of public APIs

[bart-vmware]

Address existing violations of S3956: "Generic.List" instances should not be part of public APIs in the codebase and set severity to Warning in Steeltoe.Debug.ruleset and Steeltoe.Release.ruleset.

To find existing violations, enable the rule (see above) and rebuild src/Steeltoe.All.sln to make them appear in the Output window.

To address the violations, choose from the following on a case-by-case basis:

• Fix the violation by changing the code to not violate the rule
  • Prefer collection interfaces such as ICollection<> and IDictionary<,> over arrays, list and dictionaries for inputs and outputs
    • Be hesitant with incoming IEnumerable<> parameters: the implementation may change over time, causing the need for multiple enumerations, which may result in undesired side effects
    • Consider to return unchangeable collections, such as IReadOnlyCollection<>, IReadOnlyDictionary<,> and IImmutableList<T>
• Suppress the violation in code using #pragma warning disable/restore, preceded by a justification comment if not obvious

Related to #916 and #946.