Deny unused async

[Hinton]

📔 Objective

We currently use async on several methods that don't actually require it. This PR enables the clippy rule which will warn if any async is unnecessary. It also resolves all existing usages.

⏰ 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

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (: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

[github-actions]

Checkmarx One – Scan Summary & Details – c842c5f9-7cf8-4b40-a78c-e33b54fc52be

No New Or Fixed Issues Found

[codecov]

Codecov Report

Attention: Patch coverage is 19.81982% with 89 lines in your changes missing coverage. Please review.

Project coverage is 59.64%. Comparing base (3f56e58) to head (c0099fb).

Files	Patch %	Lines
crates/bitwarden-uniffi/src/tool/sends.rs	0.00%	15 Missing ⚠️
crates/bitwarden-uniffi/src/vault/attachments.rs	0.00%	14 Missing ⚠️
crates/bw/src/main.rs	0.00%	14 Missing ⚠️
crates/bitwarden/src/mobile/tool/client_sends.rs	0.00%	9 Missing ⚠️
...s/bitwarden/src/mobile/vault/client_attachments.rs	16.66%	5 Missing ⚠️
crates/bitwarden-uniffi/src/tool/mod.rs	0.00%	4 Missing ⚠️
crates/bitwarden-uniffi/src/vault/ciphers.rs	0.00%	4 Missing ⚠️
crates/bitwarden-uniffi/src/crypto.rs	0.00%	3 Missing ⚠️
crates/bitwarden-uniffi/src/vault/folders.rs	0.00%	3 Missing ⚠️
crates/bitwarden/src/mobile/client_crypto.rs	0.00%	3 Missing ⚠️
... and 7 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #838      +/-   ##
==========================================
+ Coverage   59.24%   59.64%   +0.39%     
==========================================
  Files         186      186              
  Lines       12426    12305     -121     
==========================================
- Hits         7362     7339      -23     
+ Misses       5064     4966      -98     

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