Add support for Snippets at the Gateway level

[bjee19]

There has been some community desire for Snippets to be set at the Gateway level.

See below for some examples:
#4050 (comment)
#1200 (comment)

This could be through a SnippetsPolicy, or some sort of connection of SnippetsFilter on the Gateway.

[nginx-discourse-bot]

This issue has been mentioned on NGINX Community Forum. There might be relevant details there:

https://community.nginx.org/t/nginx-fabric-server-configuration/7760/2