feat: add Primary Backup routing policy with failover support

[fbtravi]

Add Primary Backup Routing Policy

Adds Primary Backup routing policy for automatic DNS failover with health checks.

Changes

• variables.tf - Add primary_backup to routing_policy
• main.tf - Implement primary_backup dynamic blocks
• README.md - Document routing policies (WRR, GEO, Primary Backup)

Features

• Primary/backup failover with health checks
• ILB support (L4/L7, regional/global)
• External endpoints support
• Geo-fencing and trickle ratio

Usage

routing_policy = {
  health_check = "https://www.googleapis.com/.../healthChecks/hc"
  primary_backup = {
    trickle_ratio = 0.1
    primary = { external_endpoints = ["10.0.0.1"] }
    backup_geo = [{
      location = "us-east1"
      health_checked_targets = { external_endpoints = ["10.1.0.1"] }
    }]
  }
}