Case-folding of "ProductCode" field

Brief description of your issue

The client calls a REST source and passes in the productCode in lowercase. It's doing "Exact" matching when it probably should be doing "caseInsesnsitive" as referenced in:

• https://github.com/microsoft/winget-cli-restsource/issues/166

Steps to reproduce

Add a manifest with a product code containing an upper case productCode to the REST source.
Attempt to get a matching result with the client.

Expected behavior

A match is returned

Actual behavior

No match is returned.

Environment

winget --info                                                                   in pwsh at 10:54:42
Windows Package Manager (Preview) v1.4.2161-preview
Copyright (c) Microsoft Corporation. All rights reserved.

Windows: Windows.Desktop v10.0.25212.1000
System Architecture: X64
Package: Microsoft.DesktopAppInstaller v1.19.2161.0

Logs: %LOCALAPPDATA%\Packages\Microsoft.DesktopAppInstaller_8wekyb3d8bbwe\LocalState\DiagOutputDir

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Reproduce the issue by adding a manifest with an uppercase productCode to a REST source, then query it with the lowercase value through the winget client. Trace the REST-source productCode matching entry point and verify that the query returns the manifest regardless of case. Done means the reproduced lookup returns a matching result.