Initial versioning support by codegen

[v-bilous]

This is a core change, we need to support the versioning model provided by DifHub.com. This is Re-Generation core functionality.
Initial checklist:

• support OpenAPI generation to take the latest version of the model from DifHub and store it in custom filed x-difhub-version
• Codegen to implement already generated file reads before generation.
• core change to compare the version of the already generated and take a decision about file rewrite
• database changeset generation flow – implement new changeset (with diff) instead of regeneration entire file.