Add `sqlx` database backend to VTN #256

Job	Run time
Format	6s
Audit dependencies	19s
Clippy (x86_64-unknown-linux-gnu, false)	1m 30s
Build and test (stable, x86_64-unknown-linux-gnu, ubuntu-latest, --all-features)	4m 57s
Clippy (aarch64-unknown-linux-gnu, true, -target aarch64-linux-gnu -g)	1m 24s
Build and test (msrv, x86_64-unknown-linux-gnu, ubuntu-latest, --all-features)	5m 9s
Check unused dependencies	1m 3s
Clippy (armv7-unknown-linux-gnueabihf, true, -target arm-linux-gnueabihf -mcpu=generic+v7a+vfp3-d...	1m 27s
Build and test (stable, x86_64-unknown-linux-musl, ubuntu-latest, --all-features)	5m 1s
Clippy (x86_64-unknown-linux-musl, true, -target x86_64-linux-musl)	3m 33s
Build and test (stable, aarch64-unknown-linux-gnu, ubuntu-latest, --all-features)	5m 49s
	30m 18s

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