Enable unpack/repack of .pkg containers

[mmitche]

.pkg files contain binaries that need to be signed. Today we unpack and repack these in staging, so we can sign and notarize the executables within. Expand support for this in signtool.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ellahathaway]

I will be tackling this in multiple PRs:

1. The unpack/repack tooling & associated tests. This is a separate packing that SignTool will utilize: Unpack/Repack Pkg Tooling and Tests #15205
2. The integration of the tool into SignTool: Enable Unpacking and Repacking of .pkg files and .app bundles #15206
3. Add .pkg and .app bundles as files to sign in arcade-validation:Test unpack and repack of pkgs arcade-validation#4627 (Enable arcade -sign functionality across arcade-validation builds #15181 will need to be completed first)

[ellahathaway]

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