Refactor and improve validator

[yury-palyanitsa]

The current validator is mostly a PoC which is not easily extensible, readable, and testable. Currently supported validations are the most used core validations:

• CTI syntax validation.
• Correctness of CTI inheritance chain.
• Basic correctness of schema inheritance (through inheritance implementation in JSON Schemas) similar to RAML.
• Schema validation (using gojsonschema).
• CTI presence check (in definitions and references).
• CTI reference validation.
• CTI final validation.

However, the following features are currently missing:

• Overridable validation.
• Access modifier validation.
• Foreign CTI definition check.
• More advanced/extensible schema inheritance validation.

This needs to be refactored and improved.