[200] Product compare graphQl functionality

[AlexandrKozyr]

Module thea incupsulates logic for GraphQl Product compare

Description (*)

Module includes 1 query and 2 mutations.
Automated Test will be added later by @NikZh

Fixed Issues (if relevant)

1. Product Compare #200: Product Compare

Manual testing scenarios (*)

1. Add product to Compare List by GraphQL
2. Add 2 different products to Compare List by GraphQL
3. Remove product from Compare List by GraphQL
4. Get a list of products to be compared by GrapgQL

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[naydav]

Works will be continued in
#338

[ghost]

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