[validation] add preferences to boot version and generation validations

[martinlippert]

We need to add preference settings (across Eclipse and VSCode) to switch for each of the Spring Boot version validations between ignore, info, warning, error

[nierajsingh]

Pushed most of the implementation so there should now be preferences to control the different validation conditions for boot version. However, there are some bugs to polish, namely when a user changes the preferences, it does not kick off another round of validation.

[martinlippert]

Done.