fix: handle multiple default compute classes

acorn-io/manager#1819

Adding support for multiple defaults. Will follow up with additional testing coverage.

[ ] The title of this PR would make a good line in Acorn's Release Note's Changelog
[ ] The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
[ ] All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
[ ] Commits follow contributing guidance
[ ] Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
[ ] Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
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acorn-io / runtime