Address SA1401: Fields should be private

[bart-vmware]

Address existing violations of SA1401: Fields should be private 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:

• Make the field private
• Convert the field into an auto-property (consider a get-only auto-property)
• Make the field const or readonly
• ~Suppress the violation in code using #pragma warning disable/restore, preceded by a justification comment if not obvious~

Note: less strict variants are S1104 and S2357, which solely warn on public fields. Whereas SA1401 applies to exposed fields (this includes protected, so it is more strict).