Add support for "override - BaseUri"

[jessehouwing]

For local development lots of folks override the base uri to load files from a local IIS, webpack or fiddler proxy. It would be useful when the package and publish extension could override the baseUri for UI extensions.

[wpschaub]

This would be a great enhancement, allow developers to use a baseURI for their development, which we can override, or reset, in our CI/CD pipelines.