Add `sqlx` database backend to VTN #241

Job	Run time
Format	7s
Audit dependencies	28s
Build and test (stable, x86_64-unknown-linux-gnu, ubuntu-latest, --all-features)	3m 49s
Clippy (x86_64-unknown-linux-gnu, false)	33s
Build and test (msrv, x86_64-unknown-linux-gnu, ubuntu-latest, --all-features)	4m 6s
Clippy (aarch64-unknown-linux-gnu, true, -target aarch64-linux-gnu -g)	48s
Check unused dependencies	1m 2s
Build and test (stable, x86_64-unknown-linux-musl, ubuntu-latest, --all-features)	4m 7s
Clippy (armv7-unknown-linux-gnueabihf, true, -target arm-linux-gnueabihf -mcpu=generic+v7a+vfp3-d...	51s
Build and test (stable, aarch64-unknown-linux-gnu, ubuntu-latest, --all-features)	4m 6s
Clippy (x86_64-unknown-linux-musl, true, -target x86_64-linux-musl)	38s
	20m 35s

Provide feedback