add dingding support in alertmanager

[steinliber]

Dingding is the most popular communicate tool in China like slack, but altermanager doesn't support send message to dingding robort(webhook doesn't match format in dingding ), i'd like to PR this function like slack

[stuartnelson3]

You can write a service to receive the webhook request, parse the message, and then send the correctly formatted message to dingding.

[steinliber]

ok, i will try this