Update proposal for custom attributes schema

[melnikovi]

Problem

Current schema for custom attributes doesn't support attributes that can have multiple values (checkbox, multiselect).

Solution

Requested Reviewers

[nrkapoor]

@melnikovi Does this align with the custom attribute proposal from @akaplya for Product? https://github.com/magento/architecture/pull/429

[paliarush]

1. We need to make sure that data and metadata is not mixed in the same queries because they have different life cycles. Metadata can be cached on the client and reused between queries for data.
2. Each schema modification must be covered with the example of usage. Suggestion is to split the original proposal into smaller ones, which address one problem at a time.
3. We need to add proposed schema to the running server to make sure it is syntactically correct. For interfaces we can temporarily use an existing type resolver to bypass errors.