Adding "currencies" table (#190)

* Adding "currencies" table

* Add "network_id" and a unique constraint to the currencies table

sql/01_tables_graph_database.sql

@@ -117,3 +117,15 @@ CREATE TABLE standards_events
     event_id    UUID NOT NULL REFERENCES events ON DELETE CASCADE,
     PRIMARY KEY (standard_id, event_id)
 );
+
+CREATE TABLE currencies
+(
+    id          UUID PRIMARY KEY,
+    network_id  UUID            NOT NULL REFERENCES networks ON DELETE CASCADE,
+    name        TEXT            NOT NULL,
+    symbol      VARCHAR(16)     NOT NULL,
+    address     VARCHAR(128)    NOT NULL,
+    decimals    INTEGER         NOT NULL,
+    endpoint    TEXT            NOT NULL,
+    UNIQUE(network_id, address)
+);