New feature - Add possibility to overwrite default outbound access enabled setting for subnets

[mateuszte]

Overview/Summary

This PR is a workaround to problem with default_outbound_access_enabled=true
It's allowing users to use advanced settings to overwrite default values for Out of the box subnets:
"GatewaySubnet", "AzureFirewallSubnet" and "AzureFirewallManagementSubnet" and also for custom ones.

This PR fixes/adds/changes/removes

1. Support to overwrite default settings for default_outbound_access_enabled in all Enterprise Scale subnets

Breaking Changes

1. #None

Testing Evidence

Azure/Azure-Landing-Zones#450

I tested this PR in my test environment. I was able to overwrite default settings with

                "GatewaySubnet" = {
                  default_outbound_access_enabled = false
                },
                "AzureFirewallSubnet" = {
                  default_outbound_access_enabled = false
                }

Please provide any testing evidence to show that your Pull Request works/fixes as described and planned (include screenshots, if appropriate).

As part of this Pull Request I have

• Checked for duplicate Pull Requests
• Associated it with relevant issues, for tracking and closure.
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated relevant and associated documentation.

[mateuszte]

@mateuszte please read the following Contributor License Agreement(CLA). If you agree with the CLA, please reply with the following information.

@microsoft-github-policy-service agree [company="{your company}"]

Options:

• (default - no company specified) I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer.

@microsoft-github-policy-service agree

• (when company given) I am making Submissions in the course of work for my employer (or my employer has intellectual property rights in my Submissions by contract or applicable law). I have permission from my employer to make Submissions and enter into this Agreement on behalf of my employer. By signing below, the defined term “You” includes me and my employer.

@microsoft-github-policy-service agree company="Microsoft"

Contributor License Agreement

@microsoft-github-policy-service agree

[mateuszte]

If something is missing, please let me know. It's my first PR.