feat: Allow security group rules to reference the security group created by the module

[FlorinAndrei]

Description

In the security group created by this module, allow rules to reference this security group itself as the source, in a manner similar to the self = true argument for the older aws_security_group_rule resource type.
This is done by introducing the special value "self" for the referenced_security_group_id of the SG rule resource. When this value is used, then referenced_security_group_id receives as a value the ID of the security group created by this module.

  security_group_rules = {
    ingress-self-redis = {
      type = "ingress"
      referenced_security_group_id = "self"
      description = "Allow traffic from this security group to itself."
    }
  }

Motivation and Context

aws_vpc_security_group_ingress_rule and its egress companion do not have a self argument. This is reflected in the way this module is written.
For clusters in general, it is a good idea to allow intra-cluster communication by default, for reasons such as replication, etc.
We have "self" type rules for the SGs of all clusters.

Breaking Changes

None.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[bryantbiggs]

thank you!

[antonbabenko]

This PR is included in version 1.9.0 🎉

[FlorinAndrei]

@bryantbiggs @antonbabenko I know you have other modules that could use this improvement. I'm not sure how you want to handle that. I'm a little behind the schedule right now with my projects, so I don't know how much time I'll have to spend on this topic. I'm sure you guys are far more conversant in your own code than I am, so maybe this is an easy add for you?

[bryantbiggs]

thank you - it varies on a case by case basis. I don't know of any other modules that don't have this capability but should - but happy to review any PRs where it would be of use