Update output_asset_to() and output_asset_id() to handle Output::Variable

[bitzoic]

Description

Allows for fetching of the AssetId and the to Address from an Output::Variable.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
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• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
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