Token renewal before expiry

[Pavinati]

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

astarte-client-elixir/lib/astarte/client/appengine.ex

Line 35 in 602e78a

with {:ok, jwt} <- fetch_or_generate_jwt(opts) do

This token has a default expiry of 5 minutes. If the generated client is used after the token is expired, the client does not renew the token which will make Astarte reply with 403 Forbidden.

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