Add UI for connecting to custom Elasticsearch for semantic search

Summary

Previously, semantic search could only be connected to an existing Advanced Search cluster. This merge request introduces a new vector storage settings page (admin/search/vector_storage) that allows admins to also connect to a custom Elasticsearch cluster.

When a connection is already active, submitting the form updates credentials in place rather than triggering a full reindex. Passwords are masked in the form and preserved on save unless explicitly changed.

http://localhost:3000/admin/application_settings/search#js-semantic-search-settings

Note

Changes under vector_storage_admin_settings feature flag.

Semantic search section

Before	After

Vectore storage page

Empty	Reuse Advanced Search	Custom values

Relates to #585318