Bugfixes based on BDD testing

[nkzou]

misc fixes

• re-adds default trait for models with all-optional fields
• changes templating engine to use minijinja
• nullables getting improperly propagated to child models
• api tag capitalization
• arbitrary json fields being mixed up by additionalproperties maps
• undo operation bodies skipping serialization
• inner object ordering with btreemap instead of hashmap
• add cross-version given/undo statements
• add dynamic given statements to the bdd runner
• initialization of undo api instances (for service account undo, which uses user api)
• inline oneOfs and simplify types (stop Boxing primitive types and arrays)

[skarimo]

Nice, looks good. We can rework the givens in a future PR