PRIVATE->private and PROTECTED->protected updates in Os, partial

[m-aleem]

Related Issue(s)	#3446
Has Unit Tests (y/n)	Yes - existing
Documentation Included (y/n)	N/A

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Change Description

Updates to fprime/Os to change, not all instances addressed yet:

• PRIVATE -> private
• PROTECTED -> protected
• Add friend classes, as needed

Rationale

#3446

Testing/Review Recommendations

N/A

Future Work

Additional PRs to be opened for further updates corresponding to #3446