allow optional / no egress security rule

## Describe the Feature

I would like the option to disable the egress security rule deployed by the module

```hcl
resource "aws_security_group_rule" "egress" {
count = module.this.enabled ? 1 : 0
type = "egress"
description = "Allow all egress traffic"
from_port = 0
to_port = 0
protocol = "-1"
cidr_blocks = ["0.0.0.0/0"]
security_group_id = join("", aws_security_group.default.*.id)
}
```

## Expected Behavior

Allow to disable (optional) to remove the security group. The outbound rule isn't required as security groups are statefull and the document db itself won't open a connection to the outside.

## Use Case

Scanning the file with TF_SEC throws an error

```
Problem 1

[AWS007][WARNING] Resource 'module.docdb_cluster:aws_security_group_rule.egress' defines a fully open egress security group rule.

13 | from_port = 0
14 | to_port = 0
15 | protocol = "-1"
16 | cidr_blocks = ["0.0.0.0/0"]
17 | security_group_id = join("", aws_security_group.default.*.id)
18 | }
19 |

See https://tfsec.dev/docs/aws/AWS007/ for more information.

disk i/o 114.710197ms
parsing HCL 576.897µs
evaluating values 94.868063ms
running checks 996.996µs
files loaded 23

1 potential problems detected.
```

## Describe Ideal Solution

Allow to disable (optional) to remove the security group. The outbound rule isn't required as security groups are statefull and the document db itself won't open a connection to the outside.

Start with the aws_security_group_rule.egress resource shown in the issue and inspect the module's variable definitions and security-group resources. Reproduce the TF_SEC scan, then verify that users can optionally omit the fully open egress rule while the existing default behavior remains intact.

Written by the indexing model from the issue text.