Add Variable Object to enable the usage of resource templates

[andrea-gioia]

In order to enable the reuse of resources through parametric templates we need to add a variables field to Reference Object.

Field Name	Type	Description
variables	Map[string, Variable Object]	A map between a variable name and its value. The value is used for substitution in the Reference Object.

Variable Object MUST be defined exactly like the Server Variables Object used by OpenAPI and AsyncAPI specs.