feat(sink): add Elasticsearch sink for document indexing

[psteinroe]

Adds Elasticsearch sink for indexing events as JSON documents.

Features

• Bulk document indexing for efficient batch operations
• Dynamic index routing via event metadata
• Support for configuring default index via configuration

Configuration

sink:
  type: elasticsearch
  url: http://localhost:9200
  index: events  # optional default index

Dynamic Routing

Target index can be specified per-event via metadata:

metadata = '{"index": "custom-index"}'