Improve CreateRuleGroup for tenant with no RulerMaxRuleGroupsPerTenant limit

[emanlodovice]

Describe the bug
The ruler CreateRuleGroup api still lists all rule groups under the tenant even if the RulerMaxRuleGroupsPerTenant for the tenant is 0. For huge workspaces with no limit set this results to unnecessary latency.
https://github.com/cortexproject/cortex/blob/master/pkg/ruler/api.go#L530

To Reproduce

1. Create a 3 thousand rule groups under a workspace
2. Notice that the latency of the create calls increases even if the workspace has no set limit on the number of rule groups.

Expected behavior
CreateRuleGroup should skip listing all rule groups if RulerMaxRuleGroupsPerTenant is not set.

Additional Context

[emanlodovice]

I am planning on working on this.

[yeya24]

@emanlodovice Assigned to you