Add the ability to override PluginDownloadURL if it is explicitly set on a plugin

[zbuchheit]

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Issue details

Today, you can use pluginDownloadURLOverrides to override plugins that don't have PluginDownloadURL explicitly set which is all Pulumi maintained plugins. The only problem is when attempting to use a plugin that does have PluginDownloadURL set, for example, one of the PulumiVerse plugins. In this case there is no way to override it today.

The use case is to be able to configure all plugins to be downloaded from an internally hosted solution such as Artifactory. I will open a separate issue requesting the add of a Artifactory source as well.

Affected area/feature

Plugin Discovery and Sourcing

Additional Context

It likely requires a change in PluginSpec.GetSource here but not sure if there will be some additional work needed to override the URL and get it working correctly to construct a download url root for a internal hosted solution but that is speculative.