fix: compatibility with WooGraphQL product/variation types

[justlevine]

What

This PR fixes a compatibility issue with WPGraphQL for WooCommerce, where the seo GraphQL type was not getting narrowed correctly, leading conflicts between the type and interface.

Why

Fixes #99

How

• A new Extensions namespace and requesite classes have been added to the lifecycle.
• Internally, we loop through WP_GraphQL_WooCommerce::get_enabled_{product|product_variation}_types() to register/assign the requisite GraphQL type for the seo field.
• A new INCLUDE_EXTENSIONS environment variable is used for installing woo/woographql.

Testing Instructions

Additional Info

Unit tests cannot be included until the ProductVariation.metaData interface conflict is fixed.

Checklist:

• My code is tested to the best of my abilities.
• My code follows the WordPress Coding Standards.
• My code has proper inline documentation.
• I have added unit tests to verify the code works as intended.
• The changes in this PR have been noted in CHANGELOG.md