Add jsonData support to the user/org preferences APIs

[jarben]

Why is this needed:

Plugins often need to store user preferences, it would be great if we had a key/value property such as jsonData where plugins can store user settings. This should also apply to org settings so admins can set defaults for all users.

What would you like to be added:

jsonData object similar to what we have in data sources

Who is this feature for?

Plugin users, this need was identified in many plugins such as k8s, aws, pdc,...

[jarben]

@leventebalogh, is this something your team would be responsible for? It is kind of plugin-related, but not necessarily.

[grafanabot]

@jarben please add one or more appropriate labels. Here are some tips:

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Thank you! ❤️