The Astarte JWT token is generated only once during the client initialization

Proposed solution:
Validate JWT token expiry before using it. If close to the expiration time, renew the JWT

Right now we only support the Agent part of Astarte Pairing API. We should add support for the Device part (i.e. requesting broker info, requesting and verifying certificates) so that e.g. the Astare Device SDK Elixir can start using this library instead of having to reimplement the API Client.

This should avoid being bitten by stuff like edgehog-device-manager/edgehog#479

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: astarte-platform/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

We should start gradually start documenting all public functions and modules