feat(firestore): add adapter constructor

packages/firestore/src/adapter.ts

@@ -0,0 +1,31 @@
+import { CollectionReference } from './collection-reference';
+import { Document, createDocument, getDocumentValue } from './document';
+import { Encoder } from './encoder';
+import { Storage } from './storage';
+
+export const createAdapter =
+    <A, B>(encoder: Encoder<A, B>) =>
+      (collection: CollectionReference<Document<B>>): Storage<A> => {
+        return {
+          read: async (key: string) => {
+            const snapshot = await collection.doc(key).get();
+            const document = snapshot.data();
+            if (!document) {
+              return undefined;
+            }
+            const encoded = getDocumentValue(document);
+            if (!encoded) {
+              return undefined;
+            }
+            return encoder.decode(encoded);
+          },
+          write: async (key: string, value: A) => {
+            const encoded = encoder.encode(value);
+            const doc: Document<B> = createDocument(encoded);
+            await collection.doc(key).set(doc);
+          },
+          delete: async (key: string) => {
+            await collection.doc(key).delete();
+          },
+        };
+      };