Add "Share via Slack" sharing action to embeddable panels. Use "Slack" connector created by alerting team. React components to render connectors should be available in X-Pack trigger_actions_ui plugin.

Investigate:

• Do alerting components cover our use case? Do they provide all needed props?
• Investigate how to pass interpolation variables. How to document them to users.
• Should we have templates for texts?
• Should we display "Setup sharing via Slack" if this connector has not been configured yet in alerting?

Requirements:

• Add "Share via Slack" action.
• Should open in flyout and render alerting Send-to-Slack connector.
• Pass variables to alerting React component, which can be used to interpolate into message payload.
• Should use the same license as Slack connector.

Parent issue: #60433