Added conversion API to upcast and downcast helpers

[oskarwrobel]

Suggested merge commit message (convention)

Feature (engine): Added conversion API to upcast and downcast helpers. Closes #7334.

MAJOR BREAKING CHANGE (engine): config.view callback of DowncastHelpers takes DowncastConversionApi instead of DowncastWriter. See #7334.

MAJOR BREAKING CHANGE (engine): config.model callback of UpcastHelpers takes UpcastConversionApi instead of ModelWriter. See #7334.

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[jodator]

AFAICS the only thing to change are some docs changes - I'll handle this and merge it.