[FEATURE] Certifier server

[dorzepowski]

Create certifier server that could be used later in wallet acquire certificates to acquire certificate.

1. We need to analyze https://github.com/p2ppsr/generic-certifier-backend,
2. make sure that this API is actually compatible with the request from the wallet,
3. create a new package in go-wallet-toolbox,
4. prepare some MVP server based on a standard HTTP server,

• at first without authentication – meaning without auth middleware,
• consider configuration options (especially certifier key should be configurable)
• handling only the acquire endpoint,
• add tests for the endpoint (maybe right away using AuthFetch),
• add a task for the remaining endpoints.

5. prepare main.go in cmd/ subpackage to run the server