Make SubAssetId its own type instead of using Bytes32

[Dentosal]

Draft PR to show the idea of adding a SubAssetId newtype. Contains no functional changes. Not sure if this breaks too much stuff for this to be worth it.

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

After merging, notify other teams

• Rust SDK
• Sway compiler