Trimmed apps cannot set System.Net.Security.UseManagedNtlm AppContext switch

[MichalStrehovsky]

We document this AppContext switch: https://learn.microsoft.com/en-us/dotnet/api/system.net.networkcredential?view=net-10.0

But then we hardcode a value at build time for anything that is trimmed, so it's impossible to actually set the AppContext switch directly:

runtime/src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets

Lines 59 to 60 in 2007805

	<!-- Trim managed NTLM on Linux when it's not explicitly requested -->
	<_UseManagedNtlm Condition="'$(_UseManagedNtlm)' == '' and $(RuntimeIdentifier.StartsWith('linux'))">false</_UseManagedNtlm>

I'm going to disable test that was enabled in #123055 on this issue for native AOT because the test is still using native NTLM once trimmed and it's segfaulting in native AOT outerloop runs.

Ref #90957 that had a lot of discussion.

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[wfurt]

I think it is fine if we cannot change that. But the observed behavior feels unpleasant.

[MichalStrehovsky]

If the AppContext switch is also a trimming feature switch, we typically document the associated MSBuild property, not the AppContext switch value. Then it works consistently with/without trimming.

[MichalStrehovsky]

Cc @dotnet/illink because this came up a couple times. We really need a ReadOnlyAppContext that is set at startup and cannot be changed at runtime; this is a recurring theme and a source of "stops working after I publish but works fine at F5 debug time" differences.