Improve zeroization of key buffer

[AaronFeickert]

🎟️ Tracking

None.

📔 Objective

Currently, the temporary buffer used for deriving shareable keys is manually zeroized. While documentation indicates that preceding expect calls cannot fail, this still seems brittle to future changes.

This PR places the buffer into a Zeroizing wrapper. It is still the case that zeroization may not occur on a panic, but this was already the case with the existing implementation, which would never zeroize in such a case.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation
  team

🦮 Reviewer guidelines

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• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed
  issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[CLAassistant]

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