Implement the /authz/shares API to share objects

[gprossliner]

API Endpoints

Graylog 4.0 implements the "Share" feature (https://docs.graylog.org/en/4.0/pages/permission_management.html#sharing).
This allows various entities to be shared to users and/or teams.

I need this functionality to implement a Graylog provisioning tool (Kubernetes operator), where Streams and Dashboards are created by privileged (Operator) Account, and shared with unprivileged (User) Accounts.

This API is a little bit different from others, as the "ID" of the Share entity is not autogenerated, but structured. It defines the Entity being shared (like "grn::::stream:60944c67109b552d388eaedb").

Please let me know if you want some help on this.