Support custom icons/logos for alerts and actions

[peterschretlen]

Built-in alert and action types have been using stock EUI icons, but pagerduty and logos for some planned action types are not included in EUI. EUI does support custom icons, see "custom SVGs" in EUI icon docs

When a custom icons is needed, where should these resources be located and how are they specified when registering actions or alerts in the UI?

Ideally a 3rd party can include these resources with their plugin containing their alert types and action types.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

We now have an example of how to do a custom icon: #60805.

It was decided for PagerDuty to go the custom route due to how close feature freeze is. If we are satisfied with the approach, we'll be able to close this issue as we'll have an example how to do so.

[mikecote]

Issue is now resolved, PagerDuty uses a custom icon and can be used as an example: #60805.