add google login

- refactor resolvers
- fix signup method typos

Resolves #17

server/enum/oauthProvider.go

@@ -0,0 +1,15 @@
+package enum
+
+type OAuthProvider int
+
+const (
+	GoogleProvider OAuthProvider = iota
+	GithubProvider
+)
+
+func (d OAuthProvider) String() string {
+	return [...]string{
+		"google_provider",
+		"github_provider",
+	}[d]
+}