Support $ref siblings

[leni-msft]

To align with autorest behavior https://github.com/Azure/autorest/blob/main/docs/openapi/howto/$ref-siblings.md

[mikeharder]

This should be fixed in coordination with Azure/azure-openapi-validator#637.

Siblings of $ref may be either ignored or included, depending on the spec and tool versions: Azure/azure-openapi-validator#706

As long as we're using ajv@6, I think the fix is to change this:

oav/lib/swaggerValidator/ajvSchemaValidator.ts

Line 48 in b3a9b4c

extendRefs: "fail",

From "fail" to true (https://github.com/ajv-validator/ajv/blob/v6/lib/ajv.d.ts#L180).

This option is removed in ajv@7. See https://github.com/ajv-validator/ajv/releases/tag/v7.0.0, and search for "$ref".