Preliminary changes before introducing the validation of operation extensions

⚠ Important: these changes still do not enable validation of operation extensions (only model classes for operation extensions were created, but operations models do not use them), as some work still needs to be done to get it working.

But I'm requesting it to merge in the current state as there were important change which improves the readability when validation fails like:

Before: https://gitlab.syncad.com/hive/hive/-/jobs/718276#L344

After: https://gitlab.syncad.com/hive/hive/-/jobs/718893#L352

Requires:

• !68 (merged)

Related:

• test-tools!167 (merged)
• hive!1051 (merged) (observed issue with missing extensions field for AccountUpdate2Operation)