Add CidrBlock, CidrBlockAssociationSet and Ipv6CidrBlockAssociationSet to Vpc struct.

[charlierm]

Describe the solution you'd like
Currently GetVpcById() returns a struct with no information about the CIDR associated with the VPC.

type Vpc struct {
	Id            string            // The ID of the VPC
	Name          string            // The name of the VPC
	Subnets       []Subnet          // A list of subnets in the VPC
	Tags          map[string]string // The tags associated with the VPC
	Cidr          string
	SecondaryCidr []string
}

I am writing a test which checks the IPv4 CIDR and secondary associations as well as IPv6, looking into GetVpcByID() it looks like it uses DescribeVpcs() which includes this information.

https://docs.aws.amazon.com/sdk-for-go/api/service/ec2/#Vpc

I could make the call to rather than using the helper functions in terratest but it would be nice to use them! I propose adding the following new fields to the vpc struct:

type Vpc struct {
    CidrBlock *string  // The primary IPv4 CIDR block for the VPC.
    CidrAssociations *[]string
    Ipv6CidrAssociations *[]string
}

Bit of a rough and ready proposal but you get the idea! Would be great to get some feedback, I am happy to implement this 😄

This would allow me to quickly setup a test which is something like this:

terraform.InitAndApply(t, terraformOptions)
vpcId := terraform.Output(t, terraformOptions, "vpc_id")
vpc := aws.GetVpcById(t, vpcId, "eu-west-1")
Assert.NotEmpty(t, vpc. Ipv6CidrAssociations)

Thanks!