Network Security Groups: Add rule to existing security group

[ninjarobot]

This PR closes #992

The changes in this PR are as follows:

• Network Security Groups: Add rule to existing security group

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

arm {
    add_resources [
        securityRule {
            name "web-servers"
            description "Public web server access"
            services [ "http", 80; "https", 443 ]
            add_source_tag TCP "Internet"
            add_destination_network "10.100.30.0/24"
            link_to_network_security_group existingNsg
            priority 350
        }
    ]
}