Allow adding custom mirrors to the export template manager

[CedNaru]

Describe the project you are working on

A Kotlin/Java language binding module.

Describe the problem or limitation you are having in your project

We provide users with a custom build of Godot that include our Kotlin module, similar to how Godot has a separate build with Mono.
When opening the template manager, the engine searches for a compatible template using a hardcoded link to the Godot website.
Obviously it can't find our custom build, and return a warning. Therefore, it's impossible for users to download our custom template from the editor and they have to do it manually.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Instead of using a single hardcoded address to fetch the list of templates download link, a list that can be appended by a new public API call should be iterated over so several mirror lists are possible.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

A new method could be added to EditorPlugin that allows you to directly set a link to a template or mirror.

add_template_mirror_link(String p_link)
add_template_link(String p_name, String p_link)

The first one would allow setting a link to a page returning a json with the same format as the one currently returned by https://godotengine.org/mirrorlist/4.3.stable.json

The second one would not require parsing a json, but to directly set a link to the template. It's useful in our situation, as we don't have a website to host a json but just a github link to our releases.

If this enhancement will not be used often, can it be worked around with a few lines of script?

I couldn't find a way around. The method refreshing the list of possible mirrors has a unique and hardcoded link to the Godot Website.

Is there a reason why this should be core and not an add-on in the asset library?

It's part of the editor itself, and no API exist right now to add your own mirrors or links.