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

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 87.68%. Comparing base (6ebeda5) to head (e27b0a6).
Report is 23 commits behind head on main.

Files	Patch %	Lines
Refit/ApiResponse.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1672      +/-   ##
==========================================
- Coverage   87.73%   87.68%   -0.05%     
==========================================
  Files          33       33              
  Lines        2348     2323      -25     
  Branches      294      289       -5     
==========================================
- Hits         2060     2037      -23     
  Misses        208      208              
+ Partials       80       78       -2     

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