Change CPU arch default value/validation

[un-def]

• In the fleet spec, None/missing value now means "any arch", not "must be inferred and set"
• In the run spec, None/missing value now means "any arch supported by the image", not "must be inferred and set"
• The arch inferred from the image registry is set per-JobSpec (not RunSpec) via JobConfigurator. As a consequence, resources.cpu.arch must now be set in addition to commands and user to avoid contacting the image registry (e.g., when a local image not pushed to the registry is used)
• As the arch is no longer inferred from GPU models, the image/cpu.arch field combination is only validated when cpu.arch is explicitly set to arm. A configuration that requests an NVIDIA ARM chip via gpu but sets neither image nor cpu.arch is valid now
• Relaxing resources.cpu.arch from a specific value to null is treated as a compatible change, so re-applying an unchanged configuration after a server upgrade neither redeploys service replicas nor fails the in-place update
• image: "scratch" now has a special meaning -- it forces the server to use some dummy defaults instead of contacting the image registry to extract optional field values from the image config. Used by dstack offer

Fixes: #4056