Add flag to stop egress ip being destroyed

Hardcode static ip to not be destroyable. This is not ideal, but I think better than leaving IPs to be accidentally destroyed.

Unfortunately this cannot be made conditional, as conditionals are not accepted within lifecycle attributes: https://github.com/hashicorp/terraform/issues/3116.

Additionally, the well-known limitations of prevent_destroy still apply - i.e. removing the webapp configuration from a deployment project will still cause the ip to be destroyed without warning: https://github.com/hashicorp/terraform/issues/17599.

Closes #16 (closed)