Add license headers to source files.

[MackinnonBuck]

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Description

Other .NET Foundation repositories include a license header at the top of each source file. This seems appropriate for MAUI as well.

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

Public API Changes

None

Intended Use-Case

N/A

[jfversluis]

Unless we are required to do so, we're not planning on doing this right now.

[jfversluis]

We've got word that this is required now, so let's make it happen! For context: https://github.com/dotnet/maui/pull/16770#issuecomment-1682784701