CmdPal: Add custom search engine option to Web Search extension (#43941)

## Summary of the Pull Request

This PR allows user to customize a search query in Command Palette's Web
Search built-in extension. This will also solve a problem with some
browser that doesn't handle argument in form "? <query>" as it will
allow user to specify the complete URI.

- Introduces a new text box in Web Search extension settings for
specifying a custom search engine URI
- If the text box is non-empty, the provided URI is used for queries
- If left empty, the extension defaults to previous behavior, sending
queries in the format "? query"

## Pictures? Pictures!

<img width="825" height="566" alt="image"
src="https://github.com/user-attachments/assets/fbf3d3a5-ebfe-4c16-a5f1-0d044b6f9047"
/>

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## PR Checklist

- [x] Closes: #43940 
- [x] Closes: #42867 
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.WebSearch/Helpers/ISettingsInterface.cs

@@ -18,5 +18,7 @@ public interface ISettingsInterface
 
     public IReadOnlyList<HistoryItem> HistoryItems { get; }
 
+    string CustomSearchUri { get; }
+
     public void AddHistoryItem(HistoryItem historyItem);
 }