ES|QL Views prototype

[craigtaverner]

This walking skeleton is based on the original "ESQL: View" prototype by Nik but removing the new grammar and the use of UnresolvedView, instead keeping only the REST API and the ViewService/ViewMetadata design and aim for a solution that uses the following approach:

• Use REST API for views CRUD (No views DDL in ES|QL)
• Resolve views inside UnresolvedRelation before resolving indexes (supporting name shadowing as requested by Observability)

Checklist:

• Core prototype
  • Extract key parts of Nik's original prototype
  • Write testing infrastructure to test views within csv-spec tests (single and multi-node tests)
  • Expand REST API to cover get, list, create/update, delete views
  • Add size and count limitations to stored view definitions
  • Write testing infrastructure to test views in CsvTest (csv-spec unit testing)
  • Expand scope to nested views
  • Expand scope to multiple views and views+indexes
• Enhancements
  • Prototype CCS and CPS
  • Enforce view/index name uniqueness (move view CRUD to server)
  • Control and/or restrict nesting/branching depth/width
  • Ensure views are gated behind SNAPSHOT and/or feature flag
• Split into multiple PRs for easy review
  • Multi-relation index resolution (gives CCS/CPS support)
    • Multi-relation support, a pre-requisite for views and sub-queries #136780
  • More resilient EsqlSpecTestCase
    • Re-enable some performance updates to ES|QL spec tests after resilience improvement to EsqlSpecTestCase #136781
  • View CRUD with REST
  • Rebase prototype on the above to get cleaner, simple feature PR

[elasticsearchmachine]

Hi @craigtaverner, I've created a changelog YAML for you.