improve: add use_default_ingress and use_default_egress variables

[a0s]

I need to have better SG control, but due to #27 and #9 (comment) i have to use this PR.

[RaJiska]

Hi @a0s , could you elaborate on the use cases that this PR would help solving?

[baudlord]

At least for me, this would let me define different security group rules and attach them to the security group, without Terraform trying to overwrite (and delete) them at the next apply.

We use fck-nat and transit gateways to share a NAT gateway between different VPCs, and cannot do that without #27.

[phergoualch]

I encoutered the same exact issue as @baudlord. I need better security group control to allow a larger CIDR than the VPC to connect to the NAT