Start to suppress AOT warnings

• Add a NET Standard 2.0 version of DynamicallyAccessedMembers to make it easier to decorate functions

Looking at the 24 warnings when I build with AOT:

• The first are related to constructing the source generated types, these could be solved by emitting the logic to create the types. I'm not sure how this would neatly fit into the existing library without emitting Refit.For<T>() code
• A lot of errors are related to MakeGenericType - form my understanding this is safe to use as long as the generated type is statically reachable. I believe this will always be true for refit so we can suppress this. Otherwise I could use some of my logic from feat: add aot sandbox #1969 to make remove warning.
• Code related to RefitSettings interfaces, this might be an issue.
• Query serialisation will need the user to annotate DynamicallyAccessedMembers for members that will be serialised, although refit could emit this when needed. (This has some issues)
• Warnings around json serialisation. The user can configure a JsonSerializerContext and manually add a serialisation type for every return type.
  • Refit could probably add an implementation of IHttpContentSerializer that takes JsonSerializerContext or we could add a constructor to the current one
  • Manually adding a return type to JsonSerializerContext is extremely cumbersome. I had the scuffed idea to take all the code from System.Text.Json.SourceGenerator, add it to refit, and then emit a serialiser that can handle all of the users type. This is to get around the limit around source generators not being able to call one another.
    (This should be opt in via an attribute)

Related #1389