refactor: make credentials.NewMemoryStore return an interface (#605)

1. Un-expose `credentials.MemoryStore` as it is unecessary to be public
2. Make `credentials.NewMemoryStore` return an interface instead of a
struct

Signed-off-by: Lixia (Sylvia) Lei <lixlei@microsoft.com>

Author: Wwwsylvia

registry/remote/credentials/memory_store.go

@@ -22,18 +22,18 @@ import (
 	"oras.land/oras-go/v2/registry/remote/auth"
 )
 
-// MemoryStore is a store that keeps credentials in memory.
-type MemoryStore struct {
+// memoryStore is a store that keeps credentials in memory.
+type memoryStore struct {
 	store sync.Map
 }
 
 // NewMemoryStore creates a new in-memory credentials store.
-func NewMemoryStore() *MemoryStore {
-	return &MemoryStore{}
+func NewMemoryStore() Store {
+	return &memoryStore{}
 }
 
 // Get retrieves credentials from the store for the given server address.
-func (ms *MemoryStore) Get(_ context.Context, serverAddress string) (auth.Credential, error) {
+func (ms *memoryStore) Get(_ context.Context, serverAddress string) (auth.Credential, error) {
 	cred, found := ms.store.Load(serverAddress)
 	if !found {
 		return auth.EmptyCredential, nil
@@ -42,13 +42,13 @@ func (ms *MemoryStore) Get(_ context.Context, serverAddress string) (auth.Creden
 }
 
 // Put saves credentials into the store for the given server address.
-func (ms *MemoryStore) Put(_ context.Context, serverAddress string, cred auth.Credential) error {
+func (ms *memoryStore) Put(_ context.Context, serverAddress string, cred auth.Credential) error {
 	ms.store.Store(serverAddress, cred)
 	return nil
 }
 
 // Delete removes credentials from the store for the given server address.
-func (ms *MemoryStore) Delete(_ context.Context, serverAddress string) error {
+func (ms *memoryStore) Delete(_ context.Context, serverAddress string) error {
 	ms.store.Delete(serverAddress)
 	return nil
 }

registry/remote/credentials/memory_store_test.go

@@ -40,7 +40,7 @@ func TestMemoryStore_Get_notExistRecord(t *testing.T) {
 
 func TestMemoryStore_Get_validRecord(t *testing.T) {
 	ctx := context.Background()
-	ms := NewMemoryStore()
+	ms := NewMemoryStore().(*memoryStore)
 
 	serverAddress := "registry.example.com"
 	want := auth.Credential{