Expose subnet_ids var to root module

[dimisjim]

Description

This PR exposes the subnet_ids optional var to the root module, so that it can be specified if necessary.

Documentation

Documented in the variable descriptions.

TODOs

Please ensure all of these TODOs are completed before asking for a review.

• Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
• Update the docs.
• Keep the changes backward compatible where possible.
• Run the pre-commit checks successfully.
• Run the relevant tests successfully.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.

[hashicorp-cla]

All committers have signed the CLA.