Support vendor extensions in operations

[cbornet]

This issue is for having support of swagger vendor extensions as described in:
https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#vendorExtensions
Note : the swagger core already has annotations to support the extensions

 @ApiOperation(value = "foo", extensions = {
            @Extension(properties = @ExtensionProperty(name="x-foo", value="bar"))
    })

[dilipkrish]

@cbornet This is open for extension, would you like to submit a pr with an implementation of a OperationBuilderPlugin that reads the extension properties?