Add a dedicated db crate with a basic DB trait

It's needed by the processor and tributary (coordinator).

common/db/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "serai-db"
+version = "0.1.0"
+description = "A simple database trait and backends for it"
+license = "MIT"
+repository = "https://github.com/serai-dex/serai/tree/develop/common/db"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+keywords = []
+edition = "2021"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]