feat(module/lb_internal): Adding IPv6 support to the module

[pavelrn]

Description

• Add ip_version parameter to select IPV4 or IPV6.
• Add support for IPv6 google_compute_forwarding_rule.
• Set google_compute_region_backend_service balancing_mode = "CONNECTION" explicitly due to the change in the terraform-provider-google version 6.0+

Motivation and Context

IPv6 might used for connectivity in the enterprise network. Because VM-Series is a part of the network, the deployment must support IPv6.

Closes #40.

How Has This Been Tested?

Create a load balancer as part of Common architecture.

Screenshots (if appropriate)

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.