Improving typing of PaginatedResponse.

[dennisoelkers]

Description

Motivation and Context

This PR is removing the capability of the PaginatedResponse class to define a custom key which is used to index the entity list wrapped in the response. This makes it possible to get rid of the @JsonValue method and use a simple AutoValue-class instead, enabling swagger to generate proper typings.

The benefit of this is that the response format for paginated endpoints is always identical and frontend clients can make use of the generated types.

A future refactoring could include typing the context field properly by adding a generic parameter to the class.

/jenkins-pr-deps Graylog2/graylog-plugin-enterprise#4313

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.