Use service user for role assignment and autoprovisioning

This gets us a rid of the need to configure the reva jwt secret in the
proxy. Also we no longer need to fake an internal admin user for
autoprovsioning user and/or assigning the roles to users from oidc
claims.

services/proxy/pkg/config/parser/parse.go

@@ -34,10 +34,6 @@ func ParseConfig(cfg *config.Config) error {
 }
 
 func Validate(cfg *config.Config) error {
-	if cfg.TokenManager.JWTSecret == "" {
-		return shared.MissingJWTTokenError(cfg.Service.Name)
-	}
-
 	if cfg.MachineAuthAPIKey == "" {
 		return shared.MissingMachineAuthApiKeyError(cfg.Service.Name)
 	}