list-extensions command to show experimental bases under experimental column

[f-atwi]

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Request

Rockcraft list-extensions returns a list of all extensions and their supported bases. This has no mention of any experimental bases.

$ rockcraft list-extensions 
Extension name         Supported bases
---------------------  ----------------------------------------------
django-framework       bare, ubuntu:22.04, ubuntu@22.04, ubuntu@24.04
expressjs-framework    bare, ubuntu@24.04
fastapi-framework      bare, ubuntu@24.04
flask-framework        bare, ubuntu:22.04, ubuntu@22.04, ubuntu@24.04
go-framework           bare, ubuntu@24.04
spring-boot-framework  bare, ubuntu@24.04

An example of what the proposal aims for is to have a similar result like with charmcraft where using list-extensions gives the following

$ charmcraft list-extensions 
Extension name         Supported bases    Experimental bases
---------------------  -----------------  --------------------
django-framework       ubuntu@22.04
expressjs-framework                       ubuntu@24.04
fastapi-framework                         ubuntu@24.04
flask-framework        ubuntu@22.04
go-framework                              ubuntu@24.04
spring-boot-framework                     ubuntu@24.04

The problem it solves

Provides a reliable way to show the user that an extension is experimental through the tool directly.

Provides consistent behavior between charmcraft and rockcraft.