Update kedro new add-ons flow to match design additions

[AhdraMeraliQB]

Description

As discussed with @merelcht, #2987 is just the first iteration of the implementation of the add-ons flow. As the design process continues, we may see some changes to the feature, and as a result, it's implementation. This issue is to serve as a catch-all for these changes once this design process is concluded.

Context

TODO: This issue should be fleshed out with the details of what is to be changed before being added to the sprint.

Pending

#3054

Ref: #2850

CC @amandakys

[AhdraMeraliQB]

@amandakys

Point for consideration - #2987 currently limits all/none selection to lower case only (inputting "None", for example, will throw an error), do we want to keep this restriction?

[merelcht]

Closing this in favour or #3054 and #3216