Add agent fields

Add the new Agent fields to version 7.2 of the schema - see https://github.com/Graylog2/graylog-project-illuminate/issues/4026 and https://github.com/Graylog2/graylog-project-illuminate/pull/4048

Start by reading the Agent fields request in illuminate-project-illuminate issue 4026 and the changes in pull request 4048, then locate the version 7.2 schema in this repository. Done means the new Agent fields from those references are represented in the 7.2 schema and the documentation remains consistent.

Written by the indexing model from the issue text.