Support managing Trigger Delivery Policies

[davidebriani]

The upcoming release of Astarte v1.1 will include the new resource type of Trigger Delivery Policies.
A set of CRUD APIs are available and existing policies can also be referenced by Triggers.
Supporting these operations on the astarte-client-elixir is desirable.

Further references on Trigger Delivery Policies:

1. The JSON Schema, as per Astarte v1.1.0-alpha.0, for how Trigger Policies can be defined: Delivery policy and TriggerEngine revision proposal astarte#554 (comment)
2. The Swagger file that defines the REST APIs available to manage Trigger Policies: https://github.com/astarte-platform/astarte/blob/master/apps/astarte_realm_management_api/priv/static/astarte_realm_management_api.yaml
3. Documentation about the concepts of Triggers / Trigger Policies: https://docs.astarte-platform.org/snapshot/060-triggers.html#trigger-delivery-policies
4. Documentation about using Triggers / Trigger Policies: https://docs.astarte-platform.org/snapshot/060-using_triggers.html#trigger-delivery-policies