Add managed MachO signing

[jtschuster]

Moving #107378 to a new PR since it now has little shared history.

This signer is byte-for-byte identical to the output of codesign except for padding at the end of the file and the corresponding size fields in the headers.

The MachObjectFile and CodeSignature store all the relevant headers and signature information to add/remove a signature from a Mach Object. Most other files are details regarding header format and reading files with different endianness.

The new signer uses a MemoryMappedViewAccessor instead of a Stream. Since memory mapped files can't be resized, MachObjectFile.CreateAdHocSignature() will write out the headers and load commands to the memory mapped file, but not the signature blob. The signature blob will instead be written to the destination file stream after the rest of the file is copied from the memory mapped file.

[dotnet-policy-service]

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[am11]

nit: maybe remove all StructLayout? It's default layout, there is no Pack attribute specified or no P/Invoke?

Suggested change

	using System.Runtime.InteropServices;
	namespace Microsoft.NET.HostModel.MachO;
	[StructLayout(LayoutKind.Sequential)]
	namespace Microsoft.NET.HostModel.MachO;

[jtschuster]

Removing the StructLayout causes 'unused field' warnings. I think the StructLayout makes more sense rather than suppressions and makes it clear that these are representations of unmanaged data, but am fine to change it if there's a better solution.

[am11]

We can add these binary fixtures to https://github.com/dotnet/runtime-assets/tree/main/src, e.g. Microsoft.NET.HostModel.TestData which can then be consumed as nuget package. Checking in binaries often get flagged later.

[am11]

HostActivation.Tests.SelfContainedAppLaunch.RelativeEmbeddedPath is failing on osx-x64:

System.ComponentModel.Win32Exception : An error occurred trying to start process '/Users/runner/work/1/s/artifacts/tests/host/osx.x64.Release/ha/4ishikta.mrd/HelloWorld/sub/HelloWorld' with working directory '/Users/runner/work/1/s/artifacts/bin/HostActivation.Tests/Release/net9.0'. Exec format error