IHttpContentSerializer make ToHttpContent async

[KurinnyiVadym]

It would be useful if the ToHttpContent method could be asynchronous.
In my case, the request body type T needs to be available before I can perform an asynchronous encryption step. I initially considered using a DelegatingHandler, but at that stage, the request body has already been serialised into raw JSON, and the original type T is no longer accessible.
Having an async ToHttpContent would make it possible to work with the typed request model before it’s serialised, enabling scenarios like encryption or other async preprocessing steps.