Perf: Add base64-centric C# APIs to reduce PowerShell conversion overhead

[MariusStorhaug]

Summary

Add interop-facing helper APIs that accept/return base64 where appropriate to minimize repeated PowerShell conversion work.

Why

For small payloads and high call counts, PowerShell-side UTF8/base64 conversions and binding overhead can dominate runtime.

Proposed change

• Introduce C# helper methods for common workflows (seal/open) with base64 boundaries.
• Keep existing PowerShell command signatures intact.
• Update cmdlets to delegate to helper APIs where this lowers per-call overhead.
• Preserve security checks and validation behavior.

Acceptance criteria

• Existing cmdlet contracts remain backward compatible.
• All tests pass.
• Benchmarks show improvement in looped seal/open scenarios.
• No regression in invalid input handling or error clarity.