Add explicit datasets reference to Metrics

[dmzolotarev]

Metrics reference datasets in their expressions, but don't declare these dependencies explicitly:

metrics:
  - name: revenue_per_customer
    expression:
      dialects:
        - dialect: ANSI_SQL
          expression: SUM(orders.amount) / COUNT(DISTINCT customers.id)
    # Which datasets does this use? You have to parse the SQL to find out!

This creates several issues:

1. No way to validate that referenced datasets exist
2. Unclear join paths for multi-dataset metrics
3. AI agents must parse SQL to understand dependencies
4. Can't infer dialect from datasets (Add Default Dialect at Dataset Level #16)

Proposed Solution

Add explicit datasets array to metrics definitions.

metrics:
  - name: total_revenue
    datasets: [orders]                    # Single dataset
    expression:
      dialects:
        - dialect: ANSI_SQL
          expression: SUM(orders.amount)

  - name: revenue_per_customer
    datasets: [orders, customers]         # Multiple datasets
    expression:
      dialects:
        - dialect: ANSI_SQL
          expression: SUM(orders.amount) / COUNT(DISTINCT customers.id)