feat: Add rds subnet groups for intra subnets

[Maged-Zaki]

Description

Added an optional feature for creating an RDS subnet group using the existing intra subnets in the VPC. This enables users to use RDS without the need to create a separate database subnet group

Motivation and Context

In previous projects, I had to manually create a separate subnet group using intra subnets every time I needed to deploy RDS resources. And before you say "just use database_subnet_groups", the point is, I want all services that don't require internet access (like RDS) to reside in the same isolated subnet group, such as intra_subnets. Creating and managing a separate database subnet group for this feels redundant.

Breaking Changes

No breaking changes. Just an optional functionality.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request
  I ran it but for some reason Terraform docs fails and I have no idea why.
  Edit: on second execution of the same command, Terraform docs passes but Terraform validate is stuck/frozen with no response

[github-actions]

This PR has been automatically marked as stale because it has been open 30 days
with no activity. Remove stale label or comment or this PR will be closed in 10 days

[Maged-Zaki]

I’d really appreciate it if one of the maintainers could take a look. Thanks so much!

[bryantbiggs]

thank you but we won't be adding any new subnet groups using the prior method - hopefully we'll add support for generic subnet groups that you can customize sometime soon but ... time, priorities, jobs, etc.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.