Use serde tag syntax to embed `objectType` field #56

Job	Run time
Audit dependencies	21s
Format	11s
Build and test (stable, x86_64-unknown-linux-gnu, ubuntu-latest, --all-features)	4m 49s
Clippy (x86_64-unknown-linux-gnu, false)	1m 20s
Build and test (msrv, x86_64-unknown-linux-gnu, ubuntu-latest, --all-features)	5m 37s
Clippy (aarch64-unknown-linux-gnu, true, -target aarch64-linux-gnu -g)	1m 26s
Check unused dependencies	1m 3s
Build and test (stable, x86_64-unknown-linux-musl, ubuntu-latest, --all-features)	5m 19s
Clippy (armv7-unknown-linux-gnueabihf, true, -target arm-linux-gnueabihf -mcpu=generic+v7a+vfp3-d...	1m 33s
Clippy (x86_64-unknown-linux-musl, true, -target x86_64-linux-musl)	1m 34s
Build and test (stable, aarch64-unknown-linux-gnu, ubuntu-latest, --all-features)	6m 41s
	29m 54s

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