Retrieve Customer token

[misha-kotov]

As a Magento developer, I want to retrieve the customer token in GraphQL by providing customer email/password combination so that I can effectively authorize a customer and can grab account data.

This should be similar to the POST /V1/integration/customer/token REST endpoint and implemented in GraphQL.

Acceptance Criteria

• User can successfully retrieve a customer token entirely in GraphQL by providing a correct email/password combination
• In the case of incorrect credentials, an error message is displayed:
  • The account sign-in was incorrect or your account is disabled temporarily. Please wait and try again later.
• Return type should be String
• Two arguments should be required: email: String! and password: String!
• mutation should be called generateCustomerToken

[naydav]

Merged with
#180
#184