"type" field not marked as required for derived entry resource models

[ml-evs]

As @CasperWA has pointed out, "type" does not appear as a required key for StructureResource and also LinksResource in the generated specification, because they are specified as constant fields in our models. The suggestion is to switch from Field("structures", const=True, ...) to e.g. constr(regex='^structures$'), and at the same time, we could contribute back to pydantic with a schema representation for the const string; StackOverflow suggests an enum with only one value.

Furthermore, I found that type is not listed as "required" for a StructureResource. Also, the constant value of "structures" is not provided in the specification. We could force this through the pattern property, e.g.? I even think this should be contributed upstream to FastAPI.

Originally posted by @CasperWA in #477 (comment)