[Observability] Add managedOtlpServiceUrl property to observability schema (#225635)

This change adds `managedOtlpServiceUrl` schema property that will be
populated for serverless projects and be used during onboarding for
getting the URL of the OTLP managed service.

At the moment this URL comes from the APM plugin, but it needs to be
decoupled as APM won't be available in Logs Essentials. At the same time
the property is put into the high-level `observability` plugin as it
might be used by other plugins in the future.

Author: mykolaharmash

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

@@ -81,6 +81,9 @@ const configSchema = schema.object({
   }),
   enabled: schema.boolean({ defaultValue: true }),
   createO11yGenericFeatureId: schema.boolean({ defaultValue: false }),
+  managedOtlpServiceUrl: offeringBasedSchema({
+    serverless: schema.string({ defaultValue: '' }),
+  }),
 });
 
 export const config: PluginConfigDescriptor = {