Layered Navigation Supports Multi-Select Within Field Group

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### Card
**As a** customer I can select multiple filters within a filter group **so that** I can more quickly filter my search results down for price or preferred colors or other such criteria to quickly find what I'm looking to purchase.

### Notes / Details
- For performance testing would suggest use of B2C medium profile; performant defined as no more then a 3% degradation
- Example of multi-select mockup

- Selections within filter groups are treated as an 'or'
- If easier/improves performance # results per filter selection can be dropped (similar to Amazon behavior)

### Acceptance Criteria (AC)

1. Layered navigation supports selecting multiple filters (treated as an or) within a filter group
2. Performance of un-cached category pages & search results (initial render and filter action) and pages with this change should be optimized for performance

**Original Report**: https://github.com/magento/magento2/issues/11130 by @choukalos

Start by locating the layered navigation implementation and the code paths for category pages and search results. Use the B2C medium profile to benchmark uncached initial renders and filter actions; done means multi-select works as OR within each group while staying within the stated 3% performance target.

Written by the indexing model from the issue text.