refactor: move reserved fields to the bottom of the message definitions #3637

Job	Run time
Bazel Test All	22m 11s
Bazel Build Fuzzers	5m 58s
Bazel Build Fuzzers AFL	8m 43s
Bazel Build All Config Check	7m 18s
Bazel Test Darwin x86-64	1h 35m 18s
Build IC	21m 56s
Cargo Build Release Linux	10m 40s
Cargo Clippy Linux	4m 42s
Python CI Tests	1m 33s
Build Determinism (//publish/binaries:upload, release, build-ic/release, false)	25s
Build Determinism (//publish/canisters:upload, canisters, build-ic/canisters, false)	19s
Build Determinism (//ic-os/guestos/envs/prod:upload_disk-img, guest-os/update-img, build-ic/icos/...	24s
Build Determinism (//ic-os/hostos/envs/prod:upload_update-img, host-os/update-img, build-ic/icos/...	23s
Build Determinism (//ic-os/setupos/envs/prod:upload_disk-img, setup-os/disk-img, build-ic/icos/se...	26s
	3h 0m 16s

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