[Cases] Attachment framework

[cnasikas]

Cases support different types of attachments (comments). Each type has its own schema, types, and UI representation. If a solution wants to add its own case attachment cases files need to be modified. Developers need to know the internals of the Cases architecture to be able to create their own attachment. Also, the developers of the Cases team need to review the changes every time. A framework, provided by Cases to other solutions, would facilitate these problems.

Tasks

• Create external reference type ([Cases] Add external reference attachment type #134669)
• Create UI external reference attachment registry ([Cases] Attach framework registry #134744)
• Create persistable state reference type ([Cases] Add persistable state attachment type #135452)
• Create UI persistable state attachment registry ([Cases] Persistable state UI registry #135902)
• Show on the UI the registered attachments. ([Cases] Attach framework registry #134744, [Cases] Persistable state UI registry #135902)

[elasticmachine]

Pinging @elastic/security-threat-hunting-cases (Team:Threat Hunting:Cases)

[patrykkopycinski]

It'd be cool if we could also define the UI for the component that would be responsible for adding the attachment to the Case, like Lens markdown plugin

[elasticmachine]

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

[elasticmachine]

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