Support external domains

## Describe the Feature

Currently, this module tries to configure route53. It would be nice if I could use it with domains that are not controlled by AWS.

## Expected Behavior

A clear and concise description of what you expected to happen.

## Use Case

Domain is already configured outside of AWS. Leaving domain out of the config is not possible:

```
Error: Missing required argument

on main.tf line 20, in module "ses_lambda_forwarder":
20: module "ses_lambda_forwarder" {

The argument "domain" is required, but no definition was found.
```

## Describe Ideal Solution

A clear and concise description of what you want to happen. If you don't know, that's okay.

## Alternatives Considered

n/a

Start at the required domain input in the ses_lambda_forwarder module referenced from main.tf line 20, then inspect how it configures Route53. Done should allow a domain managed outside AWS to be used without Route53 configuration while retaining the module's email-forwarding behavior.

Written by the indexing model from the issue text.