feat(core): introduce metadata

[sarahdayan]

This exposes metadata in Autocomplete.

This change aims at keeping a minimal footprint, meaning it doesn't handle fallbacks, doesn't perform any cleanups, or anything that can be post-processed.

[codesandbox-ci]

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Latest deployment of this branch, based on commit e76ddf0:

Sandbox	Source
@algolia/autocomplete-example-github-repositories-custom-plugin	Configuration
@algolia/autocomplete-example-instantsearch	Configuration
@algolia/autocomplete-example-playground	Configuration
@algolia/autocomplete-example-preview-panel-in-modal	Configuration
@algolia/autocomplete-example-react-renderer	Configuration
@algolia/autocomplete-example-starter-algolia	Configuration
@algolia/autocomplete-example-starter	Configuration
@algolia/autocomplete-example-reshape	Configuration
@algolia/autocomplete-example-vue	Configuration
@algolia/autocomplete-example-twitter-compose-with-typeahead	Configuration

See #774 (comment).