Switch API definition from WADL to newer open specification

[Tzaphkiel]

The definition of the WS API should be updated into a newer format.

Potential candidates are:

• Open API Spec 3.0.0 (old swagger 2.0 evolution)
• RAML (proprietary?!)
• Postman collections
• ...?

A potential outcome or distributed would be the equivalent set of Postman collections that could be used to test the Web Services. Another would be a Swagger or RAML documentation web site.

[sosna]

• The list of compliant APIs could be listed in the server block: https://swagger.io/docs/specification/api-host-and-base-path/
• Output can be precisely defined. Issue for XML structure-specific or CSV formats?
• Should SDMX-JSON become the default/recommended output of the RESTful API?
• Should we use the , as separator for arguments with multiple values (supported by Swagger)?
• We need to rewrite quite a few regex, apparently. All should be reviewed.
• Path parameters are always mandatory, according to the spec, and therefore, default values are considered as not applicable to path parameters, according to the docs. However, the UI seems to take the default into account... Recommended alternative is to define one operation per option (e.g. flow, flow+key, flow+key+providerRef)
• We might want to group all structural metadata under a structure resource.

[sosna]

Available in v1.5.0 of the API