Add generateInputBuilders

[martinbonnin]

Kotlin constructors with default params are nice but when compared to Builders, they lack the ability to represent the "absent" case. Builders allow 3 cases:

// present
Builder()
  .param(42)
  .build()

// null
Builder()
  .param(null)
  .build()

// absent
Builder()
  .build()

The same code in Kotlin requires wrapping the parameters in Optional which is more verbose when there are a lot of parameters (See #4747):

// present
SomeClass(param = Optional.Present(42))

// null
SomeClass(param = Optional.Present(null))

// absent (default)
SomeClass()

This PR brings back builders for those cases

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[BoD]

This is great! 🎉

The doc in plugin-configuration.mdx should be updated with the new option I guess :)

[BoD]

I guess it's one case where we could add a @Since?

[martinbonnin]

Good idea but I would only add @since once the symbol is stable?

[BoD]

oh yes good point, makes sense!

[martinbonnin]

The doc in plugin-configuration.mdx should be updated with the new option I guess :)

Right, we live in that world now. Having this duplication isn't great but you're right, I'll add it there