customers: Added ephemeral key authentication for customer retrieve (#89)
Files to modify:


**errors.rs**
  Modify existing code

**aci.rs**
  Modify existing code

**transformers.rs**
  Modify existing code

**braintree.rs**
  Modify existing code

**checkout.rs**
  Modify existing code

**stripe.rs**
  Modify existing code

**errors.rs**
  Modify existing code

**api_error_response.rs**
  Modify existing code

**helpers.rs**
  Add:
    - function: pub::make_ephemeral_key
    - function: pub::delete_ephemeral_key

**payment_response.rs**
  Modify existing code

**transformers.rs**
  Modify existing code

**ephemeral_key.rs**
  Modify existing code

**lib.rs**
  Modify existing code

**routes.rs**
  Add:
    - mod: private::ephemeral_key

**app.rs**
  Add:
    - function: pub::server
    - impl: impl EphemeralKey

**customers.rs**
  Modify existing code

**ephemeral_key.rs**
  Modify existing code

**api.rs**
  Add:
    - function: pub::authenticate_eph_key

**types.rs**
  Modify existing code

**ephemeral_key.rs**
  Modify existing code

**types.rs**
  Modify existing code