Add security group individual rule descriptions

[liamrahav]

This addresses the concern from #437. This PR adds a description into each rule block in an SG.

ingress/egress {
  from_port       = ...
  to_port         = ...
  protocol        = ...
  cidr_blocks     = ...
}

is now

ingress/egress {
  description     = "<description>"
  from_port       = ...
  to_port         = ...
  protocol        = ...
  cidr_blocks     = ...
}

There is a limitation to this approach though. When creating a rule with multiple cidr blocks or security groups, 2 separate rules are created in the AWS console, but get processed by the API/terraforming as 1 rule with 2 cidr blocks / security groups.

If you were to (1) create a rule like described above, and (2) manually edit the description of one of those rules in the AWS console, the current way terraforming is structured (using in-line rules) would not be able to preserve the changed description.

As discussed in #262, using these in-line rules allows for mixing what are really multiple rules into one rule block. When using the separate aws_security_group_rule resource, only one of cidr_blocks, ipv6_cidr_blocks, security_groups and self are allowed, which allows you to ensure that the separate descriptions for each are preserved.

This isn't a major issue as I don't think what I described impacts most use cases of terraforming, but be aware that it exists and that the ideal solution is to migrate to creating aws_security_group_rule resources alongside the main aws_security_group resource.

[liamrahav]

Tagging @nitrocode who wrote up the issue.

[coveralls]

Coverage remained the same at 100.0% when pulling 85306f6 on liamrahav:pr/sg-rule-descriptions into 67cb929 on dtan4:master.

[nitrocode]

@dtan4 @waqark3389 @liamrahav

Nice job! LGTM!

[waqarkhan3389]

ALL CHEER
MERGE MERGE MERGE MERGE MERGE

[skwp]

Please merge 🙏

[adzuci]

@liamrahav & @dtan4, this change would save us a lot of time with a large import we're currently doing, do you know if there are any blockers to merging it or when we could expect it to land? cc @arbabkhalil

[liamrahav]

@adzuci I don't have the ability to merge anything, sorry! Haven't looked at this in a while, but I suppose you can merge my branch locally for now if needed.