Feature Request: Daraja Token store

[manuelgeek]

Is your feature request related to a problem? Please describe.
The Daraja API access_tokenstays active for 1 Hour. Currently we still generate a new access token for every request, which is expensive. There's need to keep the access_token as long as it's still valid.

Describe the solution you'd like
Storing the token in a state with the GenServer

Work Involved
Getting the token, time, storing it in the state, getting the token against time, generate new one if token is older than 1 Hour