[Feature]: support reference to external Open API specification in Adapter generator

[kpanot]

Context

To generate the OPERATION_ADAPTER of an SDK, we need to parse manually the specification at generation time.
Today the code parsing the specification does support only local references (cf. the function generateOperationFinderFromSingleFile).
The purpose of this task is to support external references as well.

Proposal

There is several possible way to address this features:

• Move the generation of the OPERATION_ADAPTER in the OpenApi Generator process
• Find a library handling Json/Yaml Ref resolution and parse the resolved object
• Handle the reference resolution manually, covering the File System path resolution and Http Request