Add PostgreSQL storage backend with StorageBackend trait abstraction #25

Job	Run time
Build and Push Docker Image	4m 27s
Create Release	5s
Build Release (ubuntu-latest, aarch64-unknown-linux-gnu, diffscope, diffscope-aarch64-unknown-lin...	3m 22s
Build Release (ubuntu-latest, x86_64-unknown-linux-musl, diffscope, diffscope-x86_64-unknown-linu...	3m 17s
Build Release (macos-latest, x86_64-apple-darwin, diffscope, diffscope-x86_64-apple-darwin)	4m 4s
Build Release (macos-latest, aarch64-apple-darwin, diffscope, diffscope-aarch64-apple-darwin)	4m 4s
Build Release (windows-latest, x86_64-pc-windows-msvc, diffscope.exe, diffscope-x86_64-pc-windows...	7m 10s
Build Release (ubuntu-latest, x86_64-unknown-linux-gnu, diffscope, diffscope-x86_64-unknown-linux...	2m 59s
	29m 28s

Provide feedback