[Cases] Add the ability to create and assign playbooks to a case

[cnasikas]

Playbooks are essential for teams to reduce human error, help the team to standardize the process of incident response, improve the speed of incident processing, and ensure compliance with frameworks. Cases should provide a way for users to create their own playbooks and assign some of them to a case.

Tasks

• Add the ability to create playbooks.
• Add the ability to assign multiple playbooks to a case.
• Add the ability to mark a playbook as complete in a case.
• Add the ability to use templates like NIST.

[elasticmachine]

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

[elasticmachine]

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