feat: add Elasticsearch as data storage backend

[zhangzhenghao]

Summary

Add Elasticsearch as a data storage backend option for Gorse.

Changes

• storage/data/elasticsearch.go: New file implementing the Database interface (1715 lines)
• storage/scheme.go: Add and URL prefixes
• config/config.go: Add Elasticsearch prefix validation for data_store and cache_store
• go.mod: Add github.com/elastic/go-elasticsearch/v8 dependency
• storage/data/elasticsearch_test.go: Basic test suite

Usage

[database]
data_store = "elastic://user:password@localhost:9200/gorse"
# or with SSL:
data_store = "elastics://user:password@localhost:9200/gorse"

Implementation Details

• Implements all Database interface methods (CRUD for users, items, feedback)
• Supports batch operations and stream-based iteration
• Uses cursor-based pagination with search_after for efficient large dataset handling
• Proper index mappings for optimal query performance

Testing

• Basic test suite added (elasticsearch_test.go)
• Requires running Elasticsearch instance (set ELASTIC_URI env var for tests)