[Custom threshold] Use createLazy instead of create when initializing searchSource (elastic#213904)

## Summary

In this PR, we use a similar approach as was introduced in the ES Query
rule in this [PR](elastic#183694) for the
custom threshold rule to reduce the field_caps traffic using createLazy.
(Thanks @mikecote for pointing this out!)

||Screenshot|
|---|---|
|Create (796
ms)|![image](https://github.com/user-attachments/assets/2df8f864-bbc5-44e4-af43-7ae70f5dd2c3)|
|CreateLazy (321
ms)|![image](https://github.com/user-attachments/assets/cd9a6e51-af7e-411a-ab2e-5d7a2efd3ce5)|

### 🧪 How to test
- Enable APM locally
```
elastic.apm.active: true
elastic.apm.transactionSampleRate: 1.0
elastic.apm.environment: username
```
- Create a custom threshold rule and check its execution in
[traces](https://kibana-cloud-apm.elastic.dev/app/apm/traces?rangeFrom=now-15m&rangeTo=now)
filtered for your `username` as the environment. There should be one
with your rule name:
<img
src="https://github.com/user-attachments/assets/59274acc-3edf-4de7-8870-3b175af73523"
width=500 />
    
The timing for `_field_caps` would be more if you replace the
`createLazy` with the `create` function.

Author: maryam-saeidi

x-pack/solutions/observability/plugins/observability/server/lib/rules/custom_threshold/custom_threshold_executor.test.ts

@@ -183,7 +183,7 @@ const setup = () => {
     ...alertsServices,
     getSearchSourceClient: jest.fn().mockResolvedValue({
       ...searchSourceCommonMock,
-      create: jest.fn(() => Promise.resolve(mockedSearchSource)),
+      createLazy: jest.fn(() => Promise.resolve(mockedSearchSource)),
     }),
   };
 

x-pack/solutions/observability/plugins/observability/server/lib/rules/custom_threshold/custom_threshold_executor.ts

@@ -125,7 +125,7 @@ export const createCustomThresholdExecutor = ({
           )
         : [];
 
-    const initialSearchSource = await searchSourceClient.create(params.searchConfiguration);
+    const initialSearchSource = await searchSourceClient.createLazy(params.searchConfiguration);
     const dataView = initialSearchSource.getField('index')!;
     const { id: dataViewId, timeFieldName } = dataView;
     const runtimeMappings = dataView.getRuntimeMappings();