Issue 1671: Add MemberNotNullWhen attribute for Content property in IApiResponse<T>

[sguryev]

Potential fix for #1671

An alternative way:
new bool IsSuccessStatusCode { get; } property in the IApiResponse<T> interface with all 3 attributes above

[HavenDV]

This is similar to Breaking Change because IApiResponse<out T> is no longer IApiResponse and this may affect those using such a cast.

I suggest doing this via virtual, like here - https://stackoverflow.com/questions/72232151/membernotnullwhen-or-other-code-analysis-attributes-for-inherited-members

For some reason I was sure that it was possible to use virtual. Ok, then we should consider using new, but this can also cause some problems