feat: add PowerShell module signing example with README.md

[edwardspurlock]

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Fixes #<issue_number_goes_here> 🦕

[wangela]

Thank you for contributing a new sample with documentation. This PR contains a README but the PowerShell module that it refers to is not present. They would ideally be submitted together. If there is only one file (the .psm1 file) required to use PowerShell, consider adding that file to the root and modifying the top-level README to have a PowerShell section rather than creating a sub-directory with its own README. Also, to follow the convention of other samples in this repo, consider naming the module urlSigner.psm1

[edwardspurlock]

I have no idea what happened to the other files that should have been part of the pull request.

I can change the module name from "staticSigner" to "urlSigner" - would it be better to close this pull request and open a new one after I've made the change to the file and README ?

[edwardspurlock]

I changed the name of the module and its enclosing folder to URLsigner to match the names of the other code samples.

PowerShell modules consist of a directory that contains at least one .psm1 file with the same name:
< moduleName >/< moduleName >.psm1

I've used this structure for the URLsigner/URLsigner.psm1 module.

I created the README.md and supporting image files inside the URLsigner directory, since the directory is available to keep them separate from the other code sample files. However, do you think it would be better to remove the extended README and supporting image files for the PowerShell module, and just have the minimum amount of code (like the other code samples)?