[Cases] Assign users to a case

[cnasikas]

Summary

Users should be able to assign multiple users to a case. Specifically:

• As a user, I can assign multiple users to a case.
• As a user, I can view all cases assigned to me.
• As a user, I can view all cases assigned to multiple users.
• As a user, I can edit the assignees of a case.

To reference a user, Cases will keep the unique user profile ID in the case saved object. No other information will be kept. The reason is that user profile values (display name, email, first name, avatar, etc) can change frequently. On the single case view page, we need to fetch all user profiles of all assignees. For all cases view page (cases table) we need to fetch all assignees of all cases displayed in the table so we can show the avatars in the assignee column of the table.

Acceptance criteria:

• Assign and edit multiple users to a case.
• Filter cases by assignees.

Tasks:

• Add the assignee attribute to the SO ([ResponseOps][Cases] Add assignee field and filtering #137532).
• Extend the create API to be able to optionally add assignees to a case ([ResponseOps][Cases] Add assignee field and filtering #137532).
• Extend the update API to be able to update (add/remove) multiple assignees ([ResponseOps][Cases] Add assignee field and filtering #137532).
• Extend the find API to support filtering by assignees ([ResponseOps][Cases] Add assignee field and filtering #137532).
• Create an assignee user action ([ResponseOps][Cases] Add assignee user actions #139392).
• Create an internal endpoint to return user suggestions. The endpoint should use the suggestion function provided by the security server client. ([ResponseOps][Cases] User suggestion API #137346)
• Update UI components to assign users to a case and filter cases by assignees ([ResponseOps][Cases] Assign users on cases sidebar section #138108, [ResponseOps][Cases] Filter by assignees #139441, [Cases] Assign users when creating a case #139754).

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Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)