Introduce fixed service info type

[aniewielska]

This PR sets the value of ServiceType artifact to tes. Looks decently in ReDoc and in OpenAPI generator, but I am not sure, if this is the best possible option in OpenAPI.
Work inspired by htsget, as there seems to be no good examples in Cloud Workstream.

[kellrott]

Are we trying to get this in before 1.0?

[aniewielska]

It should not change the payload of the spec, other than by making tes the only option for the service type (artifact). But I would like to get some attention of TASC, as the solution does not look particularly elegant (> 13 lines of code just to say, that only one option is possible for one field) and would need to be repeated in all other APIs implementing Service Info.

[jb-adams]

looks good to me, essentially the same as it appears in htsget. I agree the solution doesn't look particularly elegant, but it seems to be the only way to do it in OpenAPI.

[aniewielska]

I will merge it now then, as it essentially does not change the wire format (implementations were supposed to be returning tes anyway).