dbt integration: Support multiple entities per FeatureView

YassinNouh21

Context

PR #5827 added dbt integration with single entity support.

Problem

Real-world feature engineering often requires multiple entities per FeatureView (e.g., a feature view keyed by both user_id and merchant_id). Currently only single entity column is supported.

Current Limitation

feast dbt import -m manifest.json -e driver_id  # only one entity

Proposed Enhancement

Support multiple entity columns:

# Option 1: Multiple -e flags
feast dbt import -m manifest.json -e user_id -e merchant_id

# Option 2: Comma-separated
feast dbt import -m manifest.json -e user_id,merchant_id

Implementation Notes

Update CLI to accept multiple entity columns
Create separate Entity objects for each
Pass list of entities to create_feature_view
Update code generation to handle multiple entities
Update documentation with examples

Files to Update

sdk/python/feast/cli/dbt_import.py
sdk/python/feast/dbt/mapper.py
sdk/python/feast/dbt/codegen.py
Documentation