[Cases] Add the ability to define custom fields

[cnasikas]

Each user deployment may have fields that are crucial to their business model and needs to be part of a case. Cases should support adding custom fields dynamically. Specifically, a user should be able, through the configuration page of Cases, to define up to three custom fields. The user can configure the type of the custom field (single value, multiple values, unique, toggle, etc), the key of the field, and the label of the field. Then in Cases, they can add values for the configured custom fields. Finally, the custom fields may need to be searchable.

Tasks

• Extend the cases schema to be able to accept custom fields
• Enhance the configuration page of Cases to be able to define up to three custom fields.
• Enhance the creation page and the single case view page to be able to select and set values for the custom fields configured in the configuration page.

[elasticmachine]

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

[elasticmachine]

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

[cnasikas]

Implemented by #167016