Create new user-token controller and use-case for /token endpoint

[ShreyasPrasad]

As described in step 7 of the login flow, upon querying the /token endpoint of the microservice with a valid authorization code retrieved from previous steps, a signed JWT with the following serialized properties should be returned:

1. email: the waterloo email corresponding to the user
2. id: a unique id representing the user (this could just be the UUID assigned to the user entity)

Note that based on the parameters to the /token endpoint (described here), this should happen only if the following conditions are true:

1. The provided auth code exists and is valid (i.e points to the correct clientId)
2. The EncryptedClientSecret corresponding to the clientId matches the provided client_secret.

This endpoint should appropriately sanitize query/body parameters (refer to how other use-cases achieve this through parsing middleware) and be sufficiently unit tested.