$schema should be supported

[RachitMalik12]

When $schema is provided in an open api description, validation of the open api description fails:

PS C:\Users\malikrachit\source\repos\OpenAPI.NET\artifacts\artifacts> hidi validate -d ./tests/callbacks-case8.json
fail: Microsoft.OpenApi.Hidi.Handlers.ValidateCommandHandler[0]
      => Parsing OpenAPI: ./tests/callbacks-case8.json => Detected errors
      Detected error during parsing: $schema is not a valid property at #/

This should validate as true since it is a valid description.

[baywet]

Here is the rationale for NOT erroring out.

An open API document has a schema defined, with JSON schema.
You can use that schema in any JSON document by adding a $schema property at the root, and putting the Uri of the schema.
Most ides nowadays will perform validation and auto-completion with that.
I'm saying it's a common scenario for devs, who might leave that on an OAI document.
Therefore, we should not error out when coming across it

[baywet]

After some more internal discussion:

• A JSON schema is not meant to be used for OpenAPI descriptions, that's because there are more complex validation rules that cannot be expressed with JSON schema and would lead people to believe an OpenAPI description is valid, when in fact it's not.
• We'll only warn at parsing.
• We WON'T expose a property for $schema
• Serializing a OAD which came from parsing a document which had a $schema property will be lossy