Create access token

Creates an access token that can be used to access resources in the Minna integrations API.
Please read the Authentication guide for more comprehensive information.
Note that the request body should be submitted in a Json Web Token (JWT) and signed with the private key. Minna expects the following claims inside the JWT.

// JWT claims
{
  "aud" : "minna.tech",
  "exp" : 1564494306,
  "nbf" : 1564464306,
  "clientKeyId" : "123abc",
  "clientNamespace" : "default"
}

clientNameSpace is an optional field that can be excluded. If the namespace is excluded, it will default to "default".
nbf should be set to the current time in epoch time (seconds).
exp should be the expiration for this JWT, also specified in epoch time (seconds).
The JWT is enforced to be valid for a maximum of 60 seconds, meaning that there can be a maximum of 60 seconds between nbf and exp.
Technically, this restriction should hold: (exp - nbf) <= 60.