Add in Internal AWS ELB / custom support

What does this MR do?

MR adds in the ability to provision and configure an AWS NLB for Internal Load Balancing.

Along with this it also adds in the required paths for users to provide their own External or Internal Load Balancers.

Docs included for all scenarios.

Related issues

Closes https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/220 https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/154

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up to date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: setup a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.