Add alternative versions to capabilities

[m-mohr]

If a server supports multiple API versions (e.g. 0.0.2 and 0.3.0) this could be advertised by the capabilities in GET /. This would allow clients to switch to the "best" supported version. Example:

{
  "version": "0.4.0",
  "versions": {
    "0.0.2": "http://test.org/v0",
    "0.3.1": "http://test.org/v0.3",
    "1.0.0": "http://test.org/v1.0"
  },
  "endpoints": [
    {
      "path": "/collections",
      "methods": [
        "GET"
      ]
    },
    {
      "path": "/collections/{name}",
      "methods": [
        "GET"
      ]
    },
    {
      "path": "/processes",
      "methods": [
        "GET"
      ]
    }
  ]
}

Better would be to have it one level higher as it wouldn't require changing running versions, but that's hard to specify. But maybe we can implement a "well known" URI, similar to what OpenID Connect has.

http://www.example.com/.well-known/openeo could serve:

{
  "versions": {
    "0.0.2": "http://test.org/v0",
    "0.3.1": "http://test.org/v0.3",
    "1.0.0": "http://test.org/v1.0"
  }
}

[m-mohr]

Implemented.