[My Account] Add support of Customer attributes

[naydav]

AC:

• Validation should be based on attributes(EAV) rules (for details see eav_attribute table)
  Now validation of Customer address data is hardcoded in
  \Magento\CustomerGraphQl\Model\Customer\UpdateCustomerData::execute
• Support for a case when an attribute is required and value is 0(zero)
• Only related to Frontend attributes could be loaded/created/updated
• Possibility to update partial data
• Functionality is covered by API-function tests

[naydav]

Hello @furseyev
Are you still working on the task?

[furseyev]

Hell @naydav ,

I've done the implementation in the https://github.com/magento/graphql-ce/tree/feature/271-Customer-Attributes-Support branch and needed only to add api functional tests for those.

However, merging the latest changes from 2.3-develop shows a merge conflict - one of the models I've changed is already removed, so need to figure that out and re-do the solution.

Thanks!