support unlimit keyworld when setting the resource group

[nolouch]

Feature Request

Is your feature request related to a problem? Please describe:

CREATE RESOURCE GROUP IF NOT EXISTS rg1
  RU_PER_SEC = 100
  PRIORITY = HIGH
  BURSTABLE;

Describe the feature you'd like:

supports setting RU_PER_SEC = unlimited, like the default group settings.

and also select * from information_schem.resource_groups list will shown as unlimit

[nolouch]

cc @HuSharp

[HuSharp]

/assign