Exchange Keycloak code for B2Translate tokens

Exchanges a Keycloak authorization code for B2Translate access/refresh tokens. Implements the BFF half of the OpenID Connect Authorization Code Flow.

curl -X POST "https://example.com/api/v2/auth/sso" \  -H "Content-Type: application/json" \  -d '{    "code": "eyJhbGciOi...",    "redirect_uri": "http://localhost:4200/auth/sso/callback"  }'

{  "access_token": "string",  "refresh_token": "string",  "expires_in": 0,  "refresh_expires_in": 0}