MAGE-1319 search client

[cammonro]

Summary

This PR implements the ClientInterface for backend search.

• Provides implementation for Magento\AdvancedSearch\Model\Client\ClientInterface
• Supports backend search capabilities in adminhtml context, local ConfigHelper accepts "website" scope
• Supports independent timeout configurations for backend search (distinct from legacy indexing client operations)
• Provides "Test connection" capability
• Modifies the scope handling of engine configuration
• Adds multi app ID support to backend search
• Fixed DocumentMapper sequencing bug
• Adds unit tests for DocumentMapper

NOTE: Depends on changes introduced in algolia/algoliasearch-magento-2#1850

Result

Scope specific connection testing

Error logging on failed connection

Unit tests for DocumentMapper:

[damcou]

Nice job! 🚀

I had a few comments but nothing blocking if you want to move on.