MAIN-2798 - Opsgenie slack

[Avi-Robusta]

this adds additional params to every action from a slack callback
slack_username - the user that clicked on the slack callback
slack_message - has metadata that can be used to change the slack message on button click

customPlaybooks:
 - actions:
 # adds link to opsgenie alert in slack
   - opsgenie_link_enricher:
       url_base: robusta-test-url.app.eu.opsgenie.com #needed here, url isnt in opsgenie config
   - ack_slack_opsgenie_enricher: {}
   triggers:
   - on_prometheus_alert: {}

[moshemorad]

Few comments, let me know what do you think.

[moshemorad]

Few small things.

[Avi-Robusta]

added docs