Require explicit call to async Expand::machine_id()

[hayleycall]

Summary of the Pull Request

This PR address an issue with a panic occurring due to nested Tokio runtimes when expanding {machine_id} to machine identifier. This issue is documented in #1351. The fix was to update the Expand API by modifying the function machine_id that got and set the machine identifier by making it async so that it would not need to use another Tokio runtime. Now this function runs at the time Expand is constructed rather than at the time of expanding the particular {machine_id} value when expanding each value in target_options.

PR Checklist

• Applies to work item: Expanding machine_id crashes OneFuzz agent #1351
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

Modified machine_id function, called this function at appropriate constructions of Expand. Closes #1351

Validation Steps Performed

Modified test_expand_machine_id to be async to properly test that modified function expands {machine_id} without panic.

[ranweiler]

Looking good! Added a few documentation suggestions.

Also, two remaining questions:

• Are there uses of Expand elsewhere that also need to be updated, such as in the onefuzz library crate?
• Have we manually tested this with a job that tries to expand {machine_id}?