fix: limit definitions based on use COMPASS-8941

[paula-stacho]

Main change: Standard and Expanded convertors remember which definitions are actually used, so not everything is put into the $def section in the end.
Choices made:

• keeping track of used definitions instead of traversing the schema at the end to pick the right ones -> I chose the former because the # of definitions is limited - schema is not
• the used definitions could've been encapsulated with a function, I didn't have a preference and went with OOP because the SchemaAccessor is already OOP

Note: Big chunk of the diff is due to the change to objects. Hide whitespaces helps a little.