Added NoteStore interface, mock and repository with Create method

internal/repository/notestore_test.go

@@ -0,0 +1,15 @@
+package repository_test
+
+import (
+	"git.hrafn.xyz/aether/notes/internal/models"
+)
+
+type mockNoteStore struct {
+	notes []models.Note
+}
+
+func (m *mockNoteStore) SaveNote(note models.Note) (models.Note, error) {
+	note.ID = len(m.notes) + 1
+	m.notes = append(m.notes, note)
+	return note, nil
+}