[watcher] Provide a noproxy setting for the ability to avoid proxy for specific hosts with Watcher if proxy is configured

[geekpete]

Provide the equivalent of a noproxy / NO_PROXY setting to allow scenarios where a proxy allowing only external destinations is configured to avoid traversing the proxy for internal or specific addresses that the proxy would not allow to pass through or that the user prefers not to go via the proxy for.

For example where you want to use external actions like PagerDuty/Slack requiring a proxy but also want to use Web Hooks actions against local or internal network addresses that cannot be reached if going via the configured proxy Watcher uses or preferably shouldn't go via the proxy.

So a complimentary Watcher setting to the xpack.http.proxy.host one:
https://www.elastic.co/guide/en/elasticsearch/reference/current/notification-settings.html

eg:
xpack.http.noproxy.host

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Watcher)