feat: add rfc for an external compute driver

[cheese-head]

Summary

Related Issue

Changes

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[elezar]

As a general question: Are these fields not too prescriptive? Please see #1360 where we propose a richer API which also allows some prefiltering by the gateway but a final decision by the driver.