Feat: add screen_hint param in /authorize api for explicit signup redirection (#420)

* Feat:
- Introduce screen_hint param in /authorize for explicit signup redirection

* Feat:
- Declare variable for base path and signup path
- Add social login on signup page

* Refactor:
- Update variable name for screen hint param

server/constants/oauth2.go

@@ -16,4 +16,7 @@ const (
 	ResponseTypeToken = "token"
 	// For the Implicit grant of id_token, use response_type=id_token to include an identifier token.
 	ResponseTypeIDToken = "id_token"
+
+	// Constant indicating the "signup" screen hint for customizing authentication process and redirect to a signup page.
+	ScreenHintSignUp = "signup"
 )