Add support for external references

[lscalese]

Some specifications contain external reference like ref: ../common
Add a support using validator.swagger.io tools.

[lscalese]

The service https://validator.swagger.io/ allows resolving a specification with external references.
It just needs parameters resolveFully=true and validateExternalRefs=true

The class SpecificationLoader.cls has been updated to use these parameters if a URL is used to generate the code.

Resolving external references won't be possible if we specify a file instead of a URL.

I also fixed code generation problems with special characters with property names.
Some properties contain _-@

A fix version has been pushed on OEX OpenAPI-suite 1.0.1.

I tested with 3 code generation modes the DID Manager, It looks good, no compile error with the generated code:

Set link = "https://nuts-node.readthedocs.io/en/stable/_static/didman/v1.yaml"
Set sc = ##class(dc.openapi.server.ServerAppGenerator).Generate("dc.Theo.srv",link)
Set features("simpleHttpClientOnly") = 1
Set sc = ##class(dc.openapi.client.Spec).generateApp("dc.Theo.cli", link, .features)
Set sc = ##class(dc.openapi.client.Spec).generateApp("dc.Theo.prd",link)

gen-output.txt