[Human App][Backend] Add JWT validity check

[Dzeranov]

Description
Need to add a validity check for a JWT using Reputation Oracle JWT public key

Motivation
As of now we're not checking signature of the JWT, only it's payload.

Implementation details

1. Grab Reputation Oracle JWT public key from a KVStore on a startup.
2. Cache it in Redis without TTL. (In case we change Reputation Oracle public key we just need to restart the app).