Support signing .pqx (Power Query Connector) files

[gmcchessney]

Is your feature request related to a problem? Please describe.

I need to sign a .pqx file (Power Query Connector) with an EV certificate stored in Azure Key Vault.

Describe the solution you'd like

The tool to support signing .pqx files. .pqx is an OPC file format, so signing should be similar to .vsix.

Describe alternatives you've considered

I can rename my .pqx to .vsix and sign it using this tool, then rename it back. This seems to work just fine but it's definitely a hack.

[dlemstra]

Would it be possible to provide a file so a change for this can be tested? Similar to what is done with OpenVsixSignToolTest.vsix in this project.