Setup CODEOWNERS for new plugins

[stefanalund]

Configure CODEOWNERS and define how external (non-Spotify team) can add parts of the repo.

[marcuseide]

We already have a CODEOWNERS file today, but additions to it should be part of the cli

[Rugvip]

Should probably add a question about ownership in the create-plugin command. Default to not having any owner, but if one is specified we find the CODEOWNER file and add an entry. Probably expect the CODEOWNER file to exist