Think about allowing auth_user_param point to a value from a nested structure

[barseghyanartur]

Consider the following payload:

{
   "company":{
      "name":"Royal Lafleur",
      "website":"http://google.com"
   },
   "user":{
      "first_name":"Yinthe",
      "last_name":"van Dagsburg",
      "email":"[email protected]"
   },
   "shipping_address":{
      "street":"Ivyweg",
      "house_number":"8",
      "city":"Schore",
      "postal_code":"8374IE",
      "country":"NL"
   },
   "billing_address":{
      "street":"Femkeboulevard",
      "house_number":"7",
      "city":"Hallum",
      "postal_code":"5594TA",
      "country":"NL"
   },
   "amount":1277179,
   "currency":"EUR"
}

Then something like this should be possible:

signature_to_dict(
    auth_user=payload["user"]["email"],
    secret_key=SECRET_KEY,
    extra=payload,
    auth_user_param="user.email",
)

Think about the best separator (might be the dot).