Expose external code loading with `std::execution::run_external`

[IGI-111]

Description

Fixes #1846

Checklist

• [x] I have linked to any relevant issues.
• [x] 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).
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
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