"isSession" field added to the "UpdateCustomerInput" type

[kidunot89]

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What does this implement/fix? Explain your changes.

Implements a "isSession" field for the updateCustomer mutation, when set to true, the changes made to the end-user's won't persist to the database and will instead be saved on the customer object stored in the end-user's session data. One major reason to utilize this is for shipping rate calculations where product to be shipped could be going somewhere other than the end-user's primary shipping address on file in the database.

Fixes #889

Example

updateCustomer(input: {
  isSession: true,
  shipping: {
    ...
  }
}) {
  ...
}

Does this close any currently open issues?

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Any other comments?

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