[WIP] Update 'aws_route' to handle changes in IPv6 route entries

[kula]

Allow the aws_route resource to update targets for all IPv6 destinations it can create route entries for. Currently, aws_route assumes all route targets for update are IPv4 except for a couple targets.

For example, given the following Terraform code

resource "aws_route" "default_ipv6_route" {
  route_table_id = aws_route_table.routes.id

  destination_ipv6_cidr_block = "::/0"
  network_interface_id        = aws_instance.router.primary_network_interface_id
}

this aws_route could be created, but if the network interface ever changed the provider would attempt to make the following POST to AWS

Action=ReplaceRoute&DestinationCidrBlock=&Network
InterfaceId=eni-<eni>&RouteTableId=rtb-<rtb>&Version=2016-11-15

and return the following error message:

MissingParameter: The request must contain the parameter destinationCidrBlock or destinationIpv6CidrBlock

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Output from acceptance testing:

Currently no acceptance testing, but I'm willing to tackle that once I wrap my head around it.

NOTE: I have only tested with a target of network_interface, as that's the use case I ran into.

[ewbankkit]

@kula Thanks for the contribution.
Updates to IPv6 aws_routes has been added in #14050.

[ghost]

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