feat: support setting externally created route table azure subnet

[timtalbot]

Description

Some Azure workloads use a custom route table applied to the private subnet in the VNet. The route table is defined by workload owners and not managed by our IAC. The persistent step in Pulumi creates the VNet and subnets without route table associations, but we need a way to reference an existing custom route table without Pulumi trying to overwrite or remove it.

Adds a private_subnet_route_table_id field to the network configuration that allows referencing an existing route table.

e.g.

network:
  vnet_cidr: "10.0.0.0/16"
  private_subnet_cidr: "10.0.1.0/24"
  db_subnet_cidr: "10.0.2.0/24"
  netapp_subnet_cidr: "10.0.3.0/24"
  app_gateway_subnet_cidr: "10.0.4.0/24"
  bastion_subnet_cidr: "10.0.5.0/24"
  # Custom route table for private subnet
  private_subnet_route_table_id: "/subscriptions/{subscription-id}/resourceGroups/{rg-name}/providers/Microsoft.Network/routeTables/{route-table-name}"

Issue

Category of change

• Bug fix (non-breaking change which fixes an issue)
• Version upgrade (upgrading the version of a service or product)
• New feature (non-breaking change which adds functionality)
• Build: a code change that affects the build system or external dependencies
• Performance: a code change that improves performance
• Refactor: a code change that neither fixes a bug nor adds a feature
• Documentation: documentation changes
• Breaking change (fix or feature that would cause existing functionality to not work as expected)