Allow custom schema examples in OpenAPI format

[mxck]

Hi!
I added the ability to pass examples directly through the "x-examples" field (check this #535 (comment)) and added documentation about examples in general.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mxck]

Started to check the validity of the scheme through swagger editor and realized that it needs some fine-tuning. I'll reopen the PR later.

UPD:
It turns out that only media objects have examples field. In all other cases you have to use example field.
Now in the master fields are generated incorrectly. I can fix this with a separate PR.

[mcollina]

lgtm

[mxck]

I found a error in example.
And I need to add support for response and parameters objects.

[mxck]

Please look again. I added support of examples to params, response and body.

[mcollina]

Could you make this example complete? I think we need a custom Ajv config, i.e. ajv.addKeyword() to make this work.

[mxck]

Done

[mcollina]

lgtm

[mcollina]

@climba03003 @Eomm could you take a look?

[climba03003]

LGTM.