Refactor circular ref feature

[kostyazgara]

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[ ] Regression 
[ ] Bug report
[x] Feature request
[ ] Documentation issue or request
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Current behavior

Sometimes schemas require to refer to each other

const schema1 = S.object().prop('foo', schema2);
const schema2 = S.object().prop('bar', schema1);

In this case we created special helper that resolves cirluar dependencies

const schema1 = S.object().prop('foo', createRef(() => schema2));
const schema2 = S.object().prop('bar', createRef(() => schema1));

Now circular dependencies successfully resolving with direct ref to fluent-json-schema schemas, but here some pitfalls with the approach when we extracting schema and validate payload on the fly:

1. Circular schemas are dynamic and extracting at the moment when need to validate data. In this way the ajv compiling schema at the moment when data is arrived and need to be validated
2. Swagger doesn't output the schema of dynamic ref

Expected behavior

Need to refactor ref functionality without changing the API of createRef and other additional actions from client-side

1. Compile ref schemas on startup, when we are loading all controller schemas
2. Swagger should output the schema correctly in swagger document

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Environment

Nest version: X.Y.Z

 
For Tooling issues:
- Node version: XX  
- Platform:  

Others:

[kostyazgara]

Maybe this one https://github.com/ajv-validator/ajv-keywords examples will be useful when developing forwardRef functionality