utilize HttpClientJsonExtensions.GetFromJsonAsync and alike

[ZijianFLG]

https://learn.microsoft.com/en-us/dotnet/api/system.net.http.json.httpclientjsonextensions.getfromjsonasync

HttpClientJsonExtensions

since the primary support has been shifted from newtonsoft.json to system.text.json, it may be a good time to utilize this extensions to make the generated codes even shorter.

[zijianhuang]

Not to do it because I need to support custom request headers manipulation.