Should FedCM spec have conformance requirements over IDPs

[npm1]

Currently, the IDP HTTP API section is non-normative. It could be feasible to define conformance requirements for IDPs and have that section refer to such requirements.

[kdenhartog]

#314 and #315 might be worth bringing back into scope if these requirements do come into play. Originally these were declared out of scope because this was considered non-normative. However, I don't think that if this is done that #314 and #315 MUST be brought back into scope but just rather pointing out that they could. In any case, I think it would be useful to have conformance requirements added. That was a point of confusion for me very early on when reading this spec because it wasn't clear what changes would be required of IDPs (if any) to support this API.