Use writable UserSecrets path on Apple mobile

[svick]

Fixes #132074
Fixes #60584

Summary

• ignore the unwritable HOME directory when resolving the UserSecrets path on iOS, tvOS, and MacCatalyst
• fall back to the platform-specific writable application data directory
• re-enable the UserSecrets tests previously disabled on iOS and tvOS

Testing

• dotnet build src\libraries\Microsoft.Extensions.Configuration.UserSecrets\src\Microsoft.Extensions.Configuration.UserSecrets.csproj
• dotnet build /t:test src\libraries\Microsoft.Extensions.Configuration.UserSecrets\tests\Microsoft.Extensions.Configuration.UserSecrets.Tests.csproj
• dotnet build /t:test src\libraries\Microsoft.Extensions.Configuration\tests\FunctionalTests\Microsoft.Extensions.Configuration.Functional.Tests.csproj

Note

This pull request description was drafted with GitHub Copilot.

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[svick]

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