Add support for type mapping

Similar to Kanel's customTypeMap.

One use case for this is allowing one to treat PG extension types as
some other types that they behave as (e.g., map public.ulid to String).

Fixes cornucopia-rs#246

benches/codegen.rs

@@ -17,6 +17,7 @@ fn bench(c: &mut Criterion) {
                     gen_sync: true,
                     gen_async: false,
                     derive_ser: true,
+                    config: Default::default(),
                 },
             )
             .unwrap()
@@ -32,6 +33,7 @@ fn bench(c: &mut Criterion) {
                     gen_sync: true,
                     gen_async: false,
                     derive_ser: true,
+                    config: Default::default(),
                 },
             )
             .unwrap()

benches/execution/diesel_benches.rs

@@ -164,10 +164,7 @@ pub fn bench_insert(b: &mut Bencher, conn: &mut PgConnection, size: usize) {
     };
     let insert = &insert;
 
-    b.iter(|| {
-        let insert = insert;
-        insert(conn)
-    })
+    b.iter(|| insert(conn))
 }
 
 pub fn loading_associations_sequentially(b: &mut Bencher, conn: &mut PgConnection) {

crates/cornucopia/Cargo.toml

@@ -33,3 +33,7 @@ heck = "0.4.0"
 
 # Order-preserving map to work around borrowing issues
 indexmap = "2.0.2"
+
+# Config handling
+serde = { version = "1.0.203", features = ["derive"] }
+toml = "0.8.14"