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

[sguryev]

@ChrisPulman oopsie, I'm sorry. I have forgotten to add IApiResponse interface to the ApiResponse<T> class.

Added here: https://github.com/reactiveui/refit/pull/1672/files#diff-092434a9c47d4491a15166652006ca2e0263b01b63c2029aa8653e165bfa543bR32

All tests are green now locally