Introduce cashu to host the shared types, traits and other common code (#519)

---------

Co-authored-by: thesimplekid <tsk@thesimplekid.com>

crates/cdk-common/src/database/mod.rs

@@ -0,0 +1,34 @@
+//! CDK Database
+
+#[cfg(feature = "mint")]
+mod mint;
+#[cfg(feature = "wallet")]
+mod wallet;
+
+#[cfg(feature = "mint")]
+pub use mint::Database as MintDatabase;
+#[cfg(feature = "wallet")]
+pub use wallet::Database as WalletDatabase;
+
+/// CDK_database error
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    /// Database Error
+    #[error(transparent)]
+    Database(Box<dyn std::error::Error + Send + Sync>),
+    /// DHKE error
+    #[error(transparent)]
+    DHKE(#[from] crate::dhke::Error),
+    /// NUT00 Error
+    #[error(transparent)]
+    NUT00(#[from] crate::nuts::nut00::Error),
+    /// NUT02 Error
+    #[error(transparent)]
+    NUT02(#[from] crate::nuts::nut02::Error),
+    /// Serde Error
+    #[error(transparent)]
+    Serde(#[from] serde_json::Error),
+    /// Unknown Quote
+    #[error("Unknown Quote")]
+    UnknownQuote,
+}