API Gateway missing support for webAclArn on API stages

[jrbeilke]

API Gateway supports adding a WAF to API stages in order to control access to the API, but moto is missing support for the webAclArn parameter in the API Gateway models/responses: https://docs.aws.amazon.com/apigateway/api-reference/resource/stage/#webAclArn

Looks like moto is also missing support for WAF in general, so there would need to be an initial implementation of WAF done first before this could be added (or maybe just a rudimentary validation of webAclArn to ensure it meets the webacl ARN spec for now).

[v-dev]

I have interest in getting WAF(v2) support and started an issue here: https://github.com/spulec/moto/issues/3971