Unpack/repack support for signtool for Wix5

[mmitche]

No description provided.

[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.

[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]

T-Shirt Size: M/L

There are likely challenges associated with upgrading to WiX5 given a number of toolset changes that occurred: https://github.com/orgs/wixtoolset/discussions/8560.

cc @joeloff for input on the tshirt size/cost of this work.