Fix response objects returned on Customer functions

[CassioMG]

It turns out that our Customer class functions are returning a raw request response instead of returning its data field.

This PR fixes that so it matches the expected return types for those functions as following:

• getCustomer should return object of type GetCustomerResponse
• add should return object of type AddCustomerResponse
• update should return object of type AddCustomerResponse

All the other class functions on the SDK also follow this same pattern of returning response.data field, so let's keep this same pattern for the Customer class.

It is important to notice that it is a BREAKING CHANGE for wallets out there that are using our existing Customer class.

Note: we need to merge this PR first in order to fix the tests on the Sep-7 PR