feat(kernel-headers): Use remote source and pull spec instead of local copies

[rlmenge]

Summary
Switch kernel-headers from a local spec + manual source download to pulling the upstream Fedora spec and fetching the kernel tarball from GitHub automatically.

Changes

1. Removed the local kernel-headers.spec — the 115-line vendored copy of the Fedora spec is no longer needed.
2. Changed spec type from "local" to "upstream" — azldev now pulls the spec directly from Fedora and applies overlays on top.
3. Added a source-files entry — declares the Azure Linux kernel tarball with its URL and SHA512 hash so azldev can fetch it automatically, replacing the manual curl step documented in the old TODO comment.
4. Removed the TODO workaround comment — the upstream source fetching now works correctly with overlays, so the original blocker is resolved.