Allow an end-user to be able to specify a base URL / "mirror URL" for a automatic tool installer, meaning that instead of downloading from the official repositories for tools, it would download from the specified "mirror URL". 

I understand that the instantiations of the DownloadFromUrlInstaller class use this https://updates.jenkins.io/updates/ for retrieving the URLs for the tools, from which plugins formulate some form of relative URL for the specific version/platform the user wants.

With that being said, I'm not sure how we'd go forward with either adding functionality to the above class to specify a mirror URL/creating a new option to set the mirror URL. I'm quite new to the world of Jenkins development/plugin development. My first attempt was doing it at the plugin-level by just simply replacing the base URL with the mirror URL when the performInstallation method is being called, but the plugin maintainer believes it might be better that it came from the core as a function, rather than implementing it in one plugin.

Reasons for this functionality would be for two main use cases:
1. Users in some jurisdictions may be blocked from accessing some public repositories and may need to use a public mirror such as Taobao.

2. Users of Jenkins in companies may be forced to use mirrors like Artifactory and have no access to the internet or even potential to have official repos be whitelisted.

I'd like to attempt implementing this but I don't know a huge amount about how the Update Centre JSON files and the Tool Installers and the instantiations of them work together in order to get the correct URLs. I also understand people mightn't have the time to help/give tips but any help is appreciated, would love to get this feature in.