feat: Allow adding custom routes to private route tables (#1130)

[blackapple805]

Description

This PR adds the ability to specify custom routes for private route tables by introducing a new variable private_route_table_routes. This feature allows users to add custom routes to private route tables created by the module, addressing the need outlined in issue #1130.

Changes

• Added private_route_table_routes variable to variables.tf.
• Implemented logic in main.tf to create routes in private route tables based on the new variable.
• Updated README.md to document the new variable and provide usage examples.

Usage

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "x.y.z"

  # ... other configurations ...

  private_route_table_routes = {
    for idx, rt_id in module.vpc.private_route_table_ids :
    rt_id => [
      {
        destination_cidr_block = "0.0.0.0/0"
        nat_gateway_id         = module.vpc.natgw_ids[idx]
      }
    ]
  }
}

References
Closes #1130
Checklist
 I have read the Contributing Guidelines.
 I have updated the documentation accordingly.
 I have added tests to cover my changes.
 All new and existing tests passed.

[rahul799]

@antonbabenko can we please review the PR and merge? I too want the functionality to add custom routes

[bryantbiggs]

Most of these changes are not valid nor are they relevant to what is described in the PR title

[rahul799]

ohh my bad, I didn't went through the actual changes. Let me try to fix it and raise a new PR

[bryantbiggs]

It's to easy to just @ people to ask them to do stuff without doing any work ourselves

[bryantbiggs]

closing since 90%+ of this is not valid nor applicable