UHF-12196: POC components for adding links to other instances through Linkit UI

[wktohesi]

UHF-12196

⚠️ Do not merge! This is meant for local testing and to be used as base for https://helsinkisolutionoffice.atlassian.net/browse/UHF-12647

What was done

• Added external entities storage client with mock query results, to demonstrate fetching results from an Elasticsearch endpoint. Can be developed further to actually fetch the results from a shared index.
• Added a custom external entity class and example entries in hook_entity_bundle_info_alter() and helfi_platform_config_entity_bundle_field_info_alter() on how to register the class and provide custom fields for mapping.
• Added an example entry in hook_token_info() on how to provide external entity field values via tokens (as metadata in Linkit suggestions).
• Added a substitution plugin for Linkit to use the external url when rendering content

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 65%)
21.0% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud