[ResponseOps][Cases] Custom fields - Number field type

[cnasikas]

We should add a new custom field type for numbers to increase the adoption of custom fields and empower users with more field types.

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1. Users can configure a "Number" custom field type like the current custom fields. Only one number can be provided and it can be a decimal.
2. The Kibana number formatting setting is taken into account.
3. Users can specify if the field is required and provide a default value. Validation will apply to the default value to verify that the value is a valid number.
4. Users can set the Number field type when creating a case. Validation will apply to verify that the value is a valid number.
5. Users can edit the Number field type when viewing a case. Validation will apply to verify that the value is a valid number.

[elasticmachine]

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

[elasticmachine]

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