[CodeQL] resolve issue with prototype pollution (elastic#196685)

## Summary

Relates to elastic/kibana-team#1102

Particularly addresses issues with prototype pollution

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### Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
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When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces&mdash;unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes&mdash;Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |

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Co-authored-by: Elastic Machine <[email protected]>
(cherry picked from commit 7164a34)

src/core/public/ui_settings/ui_settings_client.ts

@@ -33,7 +33,11 @@ export class UiSettingsClient implements IUiSettingsClient {
   constructor(params: UiSettingsClientParams) {
     this.api = params.api;
     this.defaults = cloneDeep(params.defaults);
-    this.cache = defaultsDeep({}, this.defaults, cloneDeep(params.initialSettings));
+    this.cache = defaultsDeep(
+      Object.create(null),
+      this.defaults,
+      cloneDeep(params.initialSettings)
+    );
 
     params.done$.subscribe({
       complete: () => {
@@ -99,7 +103,10 @@ You can use \`IUiSettingsClient.get("${key}", defaultValue)\`, which will just r
   }
 
   isDeclared(key: string) {
-    return key in this.cache;
+    return (
+      // @ts-ignore
+      (key !== '__proto__' || key !== 'constructor' || key !== 'prototype') && key in this.cache
+    );
   }
 
   isDefault(key: string) {