Enable custom threshold and inventory rules in Serverless (#170351)

Closes #170327
Relates to #168034

## Summary

This PR enables the custom threshold and inventory rules in Serverless.

|Custom threshold rule|Inventory rule|
|---|---|

|![image](https://github.com/elastic/kibana/assets/12370520/35468ded-d936-4bb1-b16b-0bcfe987e0f1)|![image](https://github.com/elastic/kibana/assets/12370520/1d1a65e0-78f5-4550-a620-9342b28ec5a9)|

Related PR: [Add the query delay mechanism to the inventory rule type
and change consumer to
Observability](#170628)

x-pack/plugins/infra/server/plugin.ts

@@ -98,7 +98,7 @@ export const config: PluginConfigDescriptor<InfraConfig> = {
       }),
       inventoryThresholdAlertRuleEnabled: offeringBasedSchema({
         traditional: schema.boolean({ defaultValue: true }),
-        serverless: schema.boolean({ defaultValue: false }),
+        serverless: schema.boolean({ defaultValue: true }),
       }),
       metricThresholdAlertRuleEnabled: offeringBasedSchema({
         traditional: schema.boolean({ defaultValue: true }),
@@ -110,7 +110,7 @@ export const config: PluginConfigDescriptor<InfraConfig> = {
       }),
       alertsAndRulesDropdownEnabled: offeringBasedSchema({
         traditional: schema.boolean({ defaultValue: true }),
-        serverless: schema.boolean({ defaultValue: false }),
+        serverless: schema.boolean({ defaultValue: true }),
       }),
     }),
   }),

x-pack/plugins/observability/server/index.ts

@@ -48,7 +48,7 @@ const configSchema = schema.object({
     }),
     thresholdRule: schema.object({
       enabled: offeringBasedSchema({
-        serverless: schema.boolean({ defaultValue: false }),
+        serverless: schema.boolean({ defaultValue: true }),
         traditional: schema.boolean({ defaultValue: true }),
       }),
     }),