Allow users to link Kibana Cases

[shanisagiv1]

Describe the feature: Kibana Cases are used for issue investigation and collaborations, in order to allow users to streamline case investigation by reviewing similar previous cases and define relationships between cases for better management and tracking, the system should support linking between cases with the following capabilities.

Describe a specific use case for the feature:

• As a user, I want the ability to link Cases as "related" to facilitate cross-functional collaboration and similarity and provide context to interconnected tasks.
• As a user, I want to review all related cases from the Case view.

Functional Requirements:

• A new field called "Linked Cases" is now available on the Case. This field is optional and can have multiple values (a valid value is an existing case ID in my space)
• When typing values, a search by Case title should be available (with auto-completion) to make it easier to search and find other cases within a case view. ( a case can't be linked to itself, but we won't enforce loops, case A is related to case B, and case B is related to Case A is a valid scenario)
• when linking a case to another, the relationship should be presented in the second case, for example: as a user, I marked Case A as "related to" case B, when reviewing case B, users should be able to see all "related cases" - including A.
• When clicking on the values, it should navigate to the linked case.
• A bulk action for linking multiple cases into a single case should be available. (should open a modal to select a single case after selecting multiple cases in the case table).
• This action (Related cases) should be logged in the activity feed.
• The new field should be available in the case table for filters and search (e.g show me all cases with that are related to case X or Y).

Non functional requirements:

• API support for any UI action.
• Nothing changes for privileges

[elasticmachine]

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

[cnasikas]

Thanks, @shanisagiv1 for the issue. These are amazing features and very helpful to our users. Linked cases and parent-child relationships between Cases are two different unrelated features. Let's create two separate issues one for each feature. Also Linked cases and Related cases are two different concepts. Wdyt about sticking to the Linked Cases terminology to avoid confusion?

[shanisagiv1]

I was wondering if it should be 2 fields as I mentioned or a single one when you can select the link type (child, related). But I agree if its 2 fields we can split it..