Add avatar to service account used for Custom flows/External agents

What does this MR do and why?

Add avatar load service for flows

With images custom flow and external agent.

References

Related to #579082

Screenshots or screen recordings

Project members	At mention in issue

How to set up and validate locally

1. Create a custom flow or external flow.
2. Add it to a group.
3. Add it to a project (select all triggers).
4. Try to @mention it.

MR acceptance checklist

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