Introduce shared sparse package identity for PowerToys (#42352)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
This pull request adds support for building, installing, and managing a
shared sparse MSIX package to grant package identity to select Win32
components in PowerToys. It introduces a new `PackageIdentity` project,
updates the installer to handle the new MSIX package during
install/uninstall, and provides developer documentation for working with
the sparse package. Additionally, new dependencies and signing rules are
included to support these changes.

**Sparse Package Identity Support**

* Added new `PackageIdentity` project to the solution for building the
sparse MSIX package, and included it in solution/project build
configurations (`PowerToys.sln`).
[[1]](diffhunk://#diff-ca837ce490070b91656ffffe31cbad8865ba9174e0f020231f77baf35ff3f811R29)
[[2]](diffhunk://#diff-ca837ce490070b91656ffffe31cbad8865ba9174e0f020231f77baf35ff3f811R54-R55)
[[3]](diffhunk://#diff-ca837ce490070b91656ffffe31cbad8865ba9174e0f020231f77baf35ff3f811R873-R880)
* Added developer documentation (`sparse-package.md`) and updated
documentation indexes to describe how to build, register, and consume
the sparse MSIX package.
[[1]](diffhunk://#diff-b4e39fb55a49c6de336d5847d75a55dd1d14840578da0ed9130f0130b61b34aaR1-R87)
[[2]](diffhunk://#diff-d0f204e503506a26ef2aa3605a8d64ac353393526fb5dcf48d4287c821f3edbcR31)
[[3]](diffhunk://#diff-430296c8d28f70d8a0164b44d7dfc30ffb1fb32466dad181947f35885b7f28d1R13)

**Installer Enhancements**

* Implemented new custom actions in the installer to install and
uninstall the `PowerToysSparse.msix` package, supporting both per-user
and machine-level scenarios (`CustomAction.cpp`, `CustomAction.def`,
`Product.wxs`).
[[1]](diffhunk://#diff-a7680a20bf0315cff463a95588a100c99d2afc53030f6e947f1f1dcaca5eefd7R597-R806)
[[2]](diffhunk://#diff-79daec0ccfcea63a2f3acb7d811b8b508529921123c754111bbccbea98b2bd74R36-R37)
[[3]](diffhunk://#diff-c12203517db7cde9ad34df9e6611457d1d3c7bc8eb7d58e06739887d3c1034afR115)
[[4]](diffhunk://#diff-c12203517db7cde9ad34df9e6611457d1d3c7bc8eb7d58e06739887d3c1034afR127)
[[5]](diffhunk://#diff-c12203517db7cde9ad34df9e6611457d1d3c7bc8eb7d58e06739887d3c1034afR149)
[[6]](diffhunk://#diff-c12203517db7cde9ad34df9e6611457d1d3c7bc8eb7d58e06739887d3c1034afR205-R210)

**Build and Dependency Updates**

* Added new NuGet package dependencies for Windows App SDK AI and
Runtime to support MSIX and sparse package features
(`Directory.Packages.props`).
* Updated signing pipeline to include the new `PowerToysSparse.msix`
artifact (`.pipelines/ESRPSigning_core.json`).
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## PR Checklist

- [ ] Closes: #xxx
- [ ] **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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or any additional comments/features here -->
## 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

---------

Co-authored-by: Gordon Lam (SH) <yeelam@microsoft.com>

.pipelines/ESRPSigning_core.json

@@ -235,7 +235,9 @@
             "*Microsoft.CmdPal.UI_*.msix",
 
             "PowerToys.DSC.dll",
-            "PowerToys.DSC.exe"
+            "PowerToys.DSC.exe",
+
+            "PowerToysSparse.msix"
           ],
             "SigningInfo": {
                 "Operations": [