Allow a generic set of keys in integrations like atlantis

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## Describe the Feature

Allow a generic set of keys in integrations like atlantis. This way we don't have to continuously update the interface whenever atlantis maintainers add/rename/delete configuration keys.

## Expected Behavior

Allow pass through generic keys from

```yaml
# atmos.yaml
integrations:
atlantis:
path: "atlantis.yaml"
config_templates:
default:
version: 3
key: value
```

```shell
atmos atlantis generate repo-config \
--config-template default \
--output-path=/dev/stdout
```

Should return

```yaml
# atlantis.yaml
version: 3
key: value
```

## references

- Created from https://github.com/cloudposse/atmos/pull/189#discussion_r964331455